posts titles unicode characters cause exceptions when running nbdev_migrate --path

# description
I am in the process of migrating my fastpages blog to nbdev and quarto. I followed the migration guide
https://nbdev.fast.ai/tutorials/blogging.html
When running  `nbdev_migrate --path posts`, the code raises an exception for unicode characters. I temporarily got around the issue by substituting my em dash with  `-`. I wanted to document this in case anyone else encountered this issue and potentially identify the function that is causing the exception

## how to reproduce
If you would like to replicate this, here's the source markdown file:
https://github.com/progressEdd/blog/blob/2e040a0a3fa86268555f00b525f33f2b86331491/_posts/2020-12-23-Investigating-RPiPlay-Apple-Airplay-on-Fedora-Linux.md
a snippet of the code it fails when the unicode character for a em dash is in the text—
```md
---
keywords: fastai
description: As a graduate student during COVID-19 some of my classes were online. I found it difficult to share my iPad screen over web conferences. At that time, I did not want to install Zoom because of the numerous <a href='https://thehackernews.com/2020/08/zoom-software-vulnerabilities.html'>security vulnerabilities</a>. Furthermore, the feature of casting my iPad screen was exclusive to Zoom. If I wanted to share my screen in other applications such as Microsoft Teams or Google Meet — I needed an alternative. Over the course of a couple months, I researched and tested multiple methods to cast my iPad screen. This blog post is the fruits of my labor.
title: Investigating RPiPlay — Apple Airplay on Fedora Linux
comments: true
nb_path: _notebooks/2020-12-23-Investigating-RPiPlay-Apple-Airplay-on-Fedora-Linux.ipynb
layout: notebook
---
...
```

When I run 
```python
uv run nbdev_migrate --path posts
Traceback (most recent call last):
  File "/mnt/sda1/Documents/development_projects/progressEdd_projects/Python-Notebooks/personal-project/blog-migration/.venv/lib64/python3.13/site-packages/nbdev/migrate.py", line 180, in nbdev_migrate
    if f.name.endswith('.md'): migrate_md(f)
                               ~~~~~~~~~~^^^
  File "/mnt/sda1/Documents/development_projects/progressEdd_projects/Python-Notebooks/personal-project/blog-migration/.venv/lib64/python3.13/site-packages/nbdev/migrate.py", line 164, in migrate_md
    txt = fp_md_fm(path)
  File "/mnt/sda1/Documents/development_projects/progressEdd_projects/Python-Notebooks/personal-project/blog-migration/.venv/lib64/python3.13/site-packages/nbdev/migrate.py", line 100, in fp_md_fm
    return _re_fm_md.sub(_dict2fm(fm), md)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/re/__init__.py", line 377, in _compile_template
    return _sre.template(pattern, _parser.parse_template(repl, pattern))
                                  ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/re/_parser.py", line 1076, in parse_template
    raise s.error('bad escape %s' % this, len(this)) from None
re.PatternError: bad escape \u at position 618 (line 10, column 26)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/mnt/sda1/Documents/development_projects/progressEdd_projects/Python-Notebooks/personal-project/blog-migration/.venv/bin/nbdev_migrate", line 12, in <module>
    sys.exit(nbdev_migrate())
             ~~~~~~~~~~~~~^^
  File "/mnt/sda1/Documents/development_projects/progressEdd_projects/Python-Notebooks/personal-project/blog-migration/.venv/lib64/python3.13/site-packages/fastcore/script.py", line 125, in _f
    return tfunc(**merge(args, args_from_prog(func, xtra)))
  File "/mnt/sda1/Documents/development_projects/progressEdd_projects/Python-Notebooks/personal-project/blog-migration/.venv/lib64/python3.13/site-packages/nbdev/migrate.py", line 181, in nbdev_migrate
    except Exception as e: raise Exception(f'Error in migrating file: {f}') from e
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```


The exact exception
```py
    return _re_fm_md.sub(_dict2fm(fm), md)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/re/__init__.py", line 377, in _compile_template
    return _sre.template(pattern, _parser.parse_template(repl, pattern))
                                  ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/re/_parser.py", line 1076, in parse_template
    raise s.error('bad escape %s' % this, len(this)) from None
re.PatternError: bad escape \u at position 618 (line 10, column 26)

The above exception was the direct cause of the following exception:
```

The issue appears to be with this function that causes the error. the `md` variable needs to add some processing to substitute the escape sequences `\u` with `\\u`. 
```py
# %% ../nbs/api/16_migrate.ipynb
def fp_md_fm(path):
    "Make fastpages front matter in markdown files quarto compliant."
    p = Path(path)
    md = p.read_text()
    fm = _fm2dict(md, nb=False)
    if fm:
        fm = _fp_convert(fm, path)
        return _re_fm_md.sub(_dict2fm(fm), md)
    else: return md 
```

Chatgpt says there are other cases as well
> - **Unicode Escapes:**  
>   - `\uXXXX`: Represents a Unicode character with 4 hex digits.  
>   - `\UXXXXXXXX`: Represents a Unicode character with 8 hex digits.  
>   - `\N{name}`: Represents a Unicode character by name.
> 
> - **Hexadecimal Escapes:**  
>   - `\xXX`: Represents a character with 2 hex digits.
> 
> - **Common Whitespace and Control Escapes:**  
>   - `\n`: Newline.  
>   - `\t`: Tab.  
>   - `\r`: Carriage return.  
>   - `\b`: Backspace (or word boundary in regex contexts).  
>   - `\f`: Form feed.  
>   - `\v`: Vertical tab.
> 
> - **Other Special Sequences in Regex:**  
>   - `\1`, `\2`, etc.: References to captured groups.  
>   - `\g<name>` or `\g<number>`: Named or numbered backreferences.

Asking it to suggest a substitution to capture all kinds of sequences for those above
```py
md = re.sub(r'(?<!\\)\\([uUNx])', r'\\\1', md)
```

### my environment

- IDE: VScodium
- Python: 3.13
   - uv
   - nbdev>=2.3.35

```bash
uv run quarto check
Quarto 1.6.42
[✓] Checking environment information...
      Quarto cache location: /home/progressedd/.cache/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.4.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.46.3: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.6.42
      Path: /home/progressedd/opt/quarto-1.6.42/bin

[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /home/progressedd/.TinyTeX/bin/x86_64-linux
      Version: 2021

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.13.2
      Path: /mnt/sda1/Documents/development_projects/progressEdd_projects/Python-Notebooks/personal-project/blog-migration/.venv/bin/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 4.4.3
      Path: /usr/lib64/R
      LibPaths:
        - /usr/lib64/R/library
        - /usr/share/R/library
      knitr: 1.33
      rmarkdown: (None)

      The rmarkdown package is not available in this R installation.
      Install with install.packages("rmarkdown")

progressEdd@codium /m/s/D/d/p/P/p/blog-migration > quarto check
Quarto 1.6.42
[✓] Checking environment information...
      Quarto cache location: /home/progressedd/.cache/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.4.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.46.3: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.6.42
      Path: /home/progressedd/opt/quarto-1.6.42/bin

[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /home/progressedd/.TinyTeX/bin/x86_64-linux
      Version: 2021

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.13.2
      Path: /usr/bin/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

      There is an unactivated Python environment in .venv. Did you forget to activate it?

[✓] Checking R installation...........OK
      Version: 4.4.3
      Path: /usr/lib64/R
      LibPaths:
        - /usr/lib64/R/library
        - /usr/share/R/library
      knitr: 1.33
      rmarkdown: (None)

      The rmarkdown package is not available in this R installation.
      Install with install.packages("rmarkdown")
```

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