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.

editor/editor_script.h

@@ -33,10 +33,10 @@
 
 #include "core/object/gdvirtual.gen.inc"
 #include "core/object/ref_counted.h"
-#include "core/object/script_language.h"
 
 class EditorInterface;
 class EditorNode;
+class Node;
 
 class EditorScript : public RefCounted {
 	GDCLASS(EditorScript, RefCounted);