Add support for Saved Games directory?

[Kromey]

Windows (since Vista) has the FOLDERID_SavedGames Known Folder (defaults to %USERPROFILE%\Saved Games). It would be nice if ProjectDirs could surface this in e.g. a saved_games_dir() method.

I don't believe Linux or MacOS has an equivalent; while you could fall back to data_dir() on those systems, that risks setting forth an opinion on a surprisingly controversial topic. It may be best instead to return an Option<&Path>, similar to runtime_dir(), which is None for MacOS and Linux so that developers can make their own decisions there. This could also return None on pre-Vista Windows systems as well.