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.

servers/physics_server_3d.h

@@ -34,7 +34,6 @@
 #include "core/io/resource.h"
 #include "core/object/class_db.h"
 #include "core/object/gdvirtual.gen.inc"
-#include "core/object/script_language.h"
 #include "core/variant/native_ptr.h"
 
 class PhysicsDirectSpaceState3D;