Better integration with xdg dirs.

[Dudemanguy]

Fixes #9147 and also supercedes #8184.

This basically does four things here.

1. Splits off the paths for macos into its own separate file (path-darwin.c).
2. Changes the internal path selection to allow for selecting different types (i.e. so you can get more than just the config directory.
3. Moves the watch_later directory to XDG_STATE_DIR (no change for the stuff that doesn't use path-unix.c).
4. Sets the cache directory to XDG_CACHE_DIR and it's used by default if applicable options aren't explicitly set (--cache-dir, --icc-shader-cache-dir, and --gpu-shader-cache-dir). On non-linux/bsd OSes, this means cache would start being saved by default in the default config dir so it's a little more invasive than the other changes.