Add SceneTree.unload_current_scene()

Provides an obvious way to unload the currently loaded scene (which is nowhere to be found in the docs).
The SceneTree.change_scene_to() method must now always provide a valid PackedScene.

Fixes #63565.

scene/main/scene_tree.h

@@ -361,6 +361,7 @@ public:
 	Error change_scene_to_file(const String &p_path);
 	Error change_scene_to_packed(const Ref<PackedScene> &p_scene);
 	Error reload_current_scene();
+	void unload_current_scene();
 
 	Ref<SceneTreeTimer> create_timer(double p_delay_sec, bool p_process_always = true, bool p_process_in_physics = false, bool p_ignore_time_scale = false);
 	Ref<Tween> create_tween();