Expose OS data directory getter methods (3.x)

[Calinou]

3.x version of #48544.

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.

I tested this PR on 3.x with the following code sample and it works as expected:

func _ready():
	print(OS.get_data_dir())  # /home/hugo/.local/share
	print(OS.get_config_dir())  # /home/hugo/.config
	print(OS.get_cache_dir())  # /home/hugo/.cache

[akien-mga]

The added doc is not alphabetically ordered.

[akien-mga]

Thanks!