Add missing return types in platformer's pause_menu.gd

[Araraura]

[Calinou]

Note that the reason these were missing before is that in Godot 3.x the type hints did not make a performance difference so we left them out of the official demos. For Godot 4.x, it's good to include them.

For future reference, see https://github.com/godotengine/godot-demo-projects/issues/868.