Fix #129 (caused by matplotlib 3.11 deprecating mpl.style.core)

[razimantv]

Solves the problem by direct implementation: Read the files and update mpl.style.library directly:

def _register_styles() -> None:
    """Register the included stylesheets in the mpl style library."""
    for path in Path(CATPPUCCIN_STYLE_DIRECTORY).glob(f"*.mplstyle"):
        stylename = path.stem
        style = rc_params_from_file(path, use_default_template=False)
        mpl.style.library.setdefault(stylename, {}).update(style)

[umbrageodotus]

Please merge asap 😭