_register_styles() fails because mpl.style.core.read_style_directory() is deprecated

matplotlib 3.11.0 has [deprecated](https://github.com/matplotlib/matplotlib/blob/main/doc/api/prev_api_changes/api_changes_3.11.0/deprecations.rst) `matplotlib.style.core`, and removed `read_style_directory()` without replacement.

As a result, `import catpuccin` fails with

```
AttributeError Traceback (most recent call last)
Cell In[2], line 1
----> 1 import catppuccin

File /venv/jupyter/lib/python3.14/site-packages/catppuccin/__init__.py:96
93 if importlib.util.find_spec("matplotlib") is not None:
94 from catppuccin.extras.matplotlib import _register_colormap_list, _register_styles
---> 96 _register_styles()
97 _register_colormap_list()

File /venv/jupyter/lib/python3.14/site-packages/catppuccin/extras/matplotlib.py:87, in _register_styles()
85 def _register_styles() -> None:
86 """Register the included stylesheets in the mpl style library."""
---> 87 catppuccin_stylesheets = mpl.style.core.read_style_directory( # type: ignore [attr-defined]
88 CATPPUCCIN_STYLE_DIRECTORY
89 )
90 mpl.style.core.update_nested_dict(mpl.style.library, catppuccin_stylesheets)
```

Start in catppuccin/extras/matplotlib.py at _register_styles(), then reproduce the failure with import catppuccin under matplotlib 3.11. Check the current Matplotlib style-registration APIs and update this integration so the import succeeds and the included stylesheets are registered without using the removed read_style_directory().

索引モデルが issue の本文から書いたものです。