Introduces `BKE_appdir_folder_caches` to get the folder that
can be used to store caches. On different OS's different folders
are used.
- Linux: `~/.cache/blender/`.
- MacOS: `Library/Caches/Blender/`.
- Windows: `(%USERPROFILE%\AppData\Local)\Blender Foundation\Blender\Cache\`.
Reviewed By: Severin
Differential Revision: https://developer.blender.org/D12822