Improve duplicate method signature diagnostic for return-type-only overloads

Improve the compiler diagnostic shown when a class defines two methods with the same name and parameter signature but different return behaviour, such as a Sub MyMethod(x As Integer) alongside a Function MyMethod(x As Integer) As Boolean.

Objo overloads methods by parameter types only; return type is not part of the method signature. The diagnostic should make that rule explicit and show the conflicting parameter signature, so users coming from Xojo understand why return-type-only overloads are rejected.