Support for loading TrueType Collection fonts

TrueType Collection fonts (commonly found as files with a .ttc extension) are now supported. Namely, Matplotlib will include these file extensions in its scan for system fonts, and will add all sub-fonts to its list of available fonts (i.e., the list from get_font_names).

From most high-level API, this means you should be able to specify the name of any sub-font in a collection just as you would any other font. Note that at this time, there is no way to specify the entire collection with any sort of automated selection of the internal sub-fonts.

In the low-level API, to ensure backwards-compatibility while facilitating this new support, a FontPath instance (comprised of a font path and a sub-font index, with behaviour similar to a str) may be passed to the font management API in place of a simple os.PathLike path. Any font management API that previously returned a string path now returns a FontPath instance instead.