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/variant/variant_setget.cpp

@@ -2385,7 +2385,7 @@ void Variant::interpolate(const Variant &a, const Variant &b, float c, Variant &
 			r_dst = a;
 		}
 			return;
-		case _RID: {
+		case RID: {
 			r_dst = a;
 		}
 			return;