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/rendering/shader_preprocessor.h

@@ -39,6 +39,7 @@
 #include "core/typedefs.h"
 
 #include "core/io/resource_loader.h"
+#include "core/object/script_language.h"
 #include "core/os/os.h"
 #include "scene/resources/shader.h"
 #include "scene/resources/shader_include.h"