stl: add type_identity to type_traits

src/entt/stl/type_traits.hpp

@@ -60,6 +60,7 @@ using std::remove_cvref_t;
 using std::remove_pointer_t;
 using std::remove_reference_t;
 using std::true_type;
+using std::type_identity;
 using std::underlying_type_t;
 
 } // namespace entt::stl