Similar idea to `inspect.Parameter`

Similar to `inspect.Signature`

- Only the most common case
- no docs
- No method body

Currently just collects the pieces, but doesn't render the final str

Also renamed constant `EXPR_ANNOTATION` -> `INPUT_ANNOTATION`

- Contains all the functionality
- Needs a lot of tidying up

- Uses the same config as `indent_docstring`
- Can't truly be `numpydoc` though without a parameters section

See https://docutils.sourceforge.io/docs/user/rst/quickref.html#external-hyperlink-targets

- Renamed `name_safe` -> `title`
- Perform an eager pass to extract replacement pairs

#3600 (comment)

I've identified multiple kinds of overloads.
If we want to further improve accuracy, then these would need to be considered separately

- Separates the token vs string iteration
- Improve doc, w/ example

- Renaming
- Allow refreshing on any call
- Auto refresh if first call did not

- Already general enough to be used in other areas of `tools`.
- E.g. for `vega-lite` docs using `true`, `false`, `null` -> `True`, `False`, `None`

…erable iterator

- Was previously being handled in multiple places
- Now the correct name is set during the first access to `self.name`

Little bit easier to see how they relate

There are more things I want to change, but ideally keep `write_expr_module` consistent

- Need to do some experimenting with a new test suite for `expr`
- Temporarily isolating this to `expr.dummy.py`
- Will later replace `expr.__init__.py`

Need to make a change here to start a GH thread

#3600 (comment)

- Combined some single use functions
- `signature_n_params`
  - Collect the signature here, since the function is prefixed with `signature_`
  - Use a cheaper method of calculating number of target params
- Reordered, renamed, prepare `test_expr_methods` for the new `expr` type

#3600 (comment)

`ruff` was warning that `TYPE_CHECKING` was unused

#3600 (comment)

#3600 (comment)

- `IntoExpression` change I'll add in a new fix PR
- `OperatorMixin` todo needs an issue

Also some rearranging in `vega_expr.py`

#3600 (comment), #3600 (comment)

Also moved some static content to template and simplified

#3600 (comment), #3600 (comment), #3600 (comment)

The `EXPR_` prefix is meaningless when all use it

#3600 (comment)

- Both have a `.from_..()` iterator classmethod
- Both output via `.render()`

#3600 (comment)

#3600 (comment)

- Moves a replacement that previously occured in two places (rendering & signatuyre parsing)
- Now the tokens returned by `read_ast_tokens` do not contain this at all

#3600 (comment)

Step is unrelated to parsed attributes of a definition

This method was always intended to be temorary and is no longer helpful

Related to (but doesn't resolve) #3600 (comment)