Feature request: API to get paths for cache folder and preferences folder

[impiaaa]

Currently, SDL has SDL_GetPrefPath to get the path of a general-purpose writable directory, that returns:

• %APPDATA%\Roaming\CompanyName\AppName on Windows
• ~/Library/Application Support/AppName on macOS
• ~/.local/share/AppName on Linux/XDG
• /data/data/AppName/files on Android

It would also be handy to get more specific paths, like for storing cache data:

• %APPDATA%\Local\CompanyName\AppName on Windows (I think so anyway, I'm not experienced here)
• ~/Library/Cache/AppName on macOS
• ~/.cache/AppName on Linux/XDG
• /data/data/AppName/cache on Android (exposed through SDL_GetAndroidCachePath in SDL)

or preferences or configuration data:

• ~/Library/Preferences/AppName on macOS
• ~/.config/AppName on Linux/XDG
• (I think there is none on Windows because you're expected to use the registry for that)

SDL also has SDL_GetUserFolder that these could be added to, but SDL_GetUserFolder is currently just for user-visible folders like Documents and Downloads, whereas the cache and prefs are hidden by default in the OSs listed above. A new API for this could help fix #4447 and make #3336 more explicit/transparent.