Expose OS data directory getter methods

[Calinou]

master version of #49732.

This can be used by editor plugins and non-game applications to store data in the correct directories according to the XDG Base Directory specification.

Testing code

func _ready():
	print(OS.get_data_dir())
	print(OS.get_config_dir())
	print(OS.get_cache_dir())

Output (Linux)

/home/hugo/.local/share
/home/hugo/.config
/home/hugo/.cache

This closes godotengine/godot-proposals#2696.

[akien-mga]

Needs a rebase.

[akien-mga]

Thanks!