Extract ScriptInstance to simplify includes

This allows to include script_instance.h directly in the
generated gdvirtual.gen.inc, and remove excessive includes
from the codebase.

This should also allow Resource to use GDVIRTUAL macros,
which wasn't possible previously due to a circular dependency.

modules/multiplayer/scene_cache_interface.h

@@ -33,6 +33,7 @@
 
 #include "scene/main/multiplayer_api.h"
 
+class Node;
 class SceneMultiplayer;
 
 class SceneCacheInterface : public RefCounted {