feat: add write_yaml_metadata_block()

[juleswg23]

Description

We replicate the behavior of the quarto-r equivalent: https://quarto-dev.github.io/quarto-r/reference/write_yaml_metadata_block.html

This intends to support one path for injecting recipients into email document metadata, to support quarto-dev/quarto-cli#14098

Manual testing

We can verify this behavior works by rendering the below document

---
title: "Dynamic Metadata Example"
format: html
---

## Basic Usage

Set metadata dynamically from Python:

```{python}
#| output: asis
from quarto import write_yaml_metadata_block

write_yaml_metadata_block(
  admin=True,
  user_level="advanced",
  version="1.0"
)
```

## Using Conditional Content

Now we can use the dynamically set metadata:

::: {.content-visible when-meta="admin"}
This content is only visible when admin=true!
:::

::: {.content-hidden when-meta="admin"}
This content is hidden when admin=true.
:::

## Dynamic Computation

Compute values in Python and use them as metadata:

```{python}
#| output: asis
from quarto import write_yaml_metadata_block
import datetime

today = datetime.date.today()
is_weekend = today.weekday() >= 5

write_yaml_metadata_block(
  date=str(today),
  is_weekend=is_weekend,
  day_name=today.strftime("%A")
)
```

::: {.content-visible when-meta="is_weekend"}
It's the weekend!
:::

::: {.content-hidden when-meta="is_weekend"}
It's a weekday.
:::