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`.

modules/mono/csharp_script.cpp

@@ -477,7 +477,7 @@ static String variant_type_to_managed_name(const String &p_var_type_name) {
 		Variant::COLOR,
 		Variant::STRING_NAME,
 		Variant::NODE_PATH,
-		Variant::_RID,
+		Variant::RID,
 		Variant::CALLABLE
 	};