Remove RES and REF typedefs in favor of spelled out Ref<>

These typedefs don't save much typing compared to the full `Ref<Resource>`
and `Ref<RefCounted>`, yet they sometimes introduce confusion among
new contributors.

modules/gridmap/grid_map.cpp

@@ -797,7 +797,7 @@ void GridMap::clear() {
 	clear_baked_meshes();
 }
 
-void GridMap::resource_changed(const RES &p_res) {
+void GridMap::resource_changed(const Ref<Resource> &p_res) {
 	_recreate_octant_data();
 }