Add stub for Styler.map_index

[cquick01]

• Closes #xxxx (Replace xxxx with the Github issue number)

  Couldn't find an issue, but it might belong in Version 2.0 Compatibility Tracker #624. This is for this method https://github.com/pandas-dev/pandas/blob/5232bee8df3b57766c44b62152aa3fdd24e40ada/pandas/io/formats/style.py#L2018

  The function signature in code is

     def map_index(
          self,
          func: Callable,
          axis: AxisInt | str = 0,
          level: Level | list[Level] | None = None,
          **kwargs,
      ) -> Styler:

  but this looks to be the same as how Styler.apply_index is defined in code, but the stub is a little different so I just copied that instead

• Tests added: Adds test_styler.test_map_index, similarly adapted from test_apply_index

[Dr-Irv]

Based on the docs, I think this should be Callable[[Scalar], str]

[cquick01]

Thanks, I updated to Callable[[Scalar], str | None] to match the docs

[Dr-Irv]

The tests should be modified to something similar to what you see in the docs for map_index()

[cquick01]

Updated to show the first example from the docs. The second one showed the following error

    def f1(s: Scalar):
        return "background-color: yellow;" if "x" in s else None
                                              ~~~~~~~~
Unsupported right operand type for in ("str | bytes | date | datetime | timedelta | datetime64 | timedelta64 | bool | int | float | Timestamp | Timedelta | complex")

[Dr-Irv]

Thanks @cquick01