Variant: Rename Type::_RID to Type::RID

The underscore prefix was used to avoid the conflict between the `RID` class
name and the matching enum value in `Variant::Type`.

This can be fixed differently by prefixing uses of the `RID` class in `Variant`
with the scope resolution operator, as done already for `AABB`.

core/io/packed_data_container.cpp

@@ -246,7 +246,7 @@ uint32_t PackedDataContainer::_pack(const Variant &p_data, Vector<uint8_t> &tmpd
 
 		} break;
 		// misc types
-		case Variant::_RID:
+		case Variant::RID:
 		case Variant::OBJECT: {
 			return _pack(Variant(), tmpdata, string_cache);
 		} break;