Expose TileSet parsing more directly

[Cleptomania]

Currently TileSet parsing is a fairly internal function not intended for general consumption, while it is possible to use, it's not necessarily designed for it.

There are valid use-cases where a user may want to just parse a TileSet and not a full map, and PyTiled Parser's API doesn't lend itself well to doing that.

[Cleptomania]

Our test suite is using TileSet parsing directly, so looking at how that works can give an idea of how to do that with the current API.

[Cleptomania]

This is available in 2.2.3 with the pytiled_parser.parse_tileset function.