Support generic type T to extend multiple classes

[nathnaeld]

Currently, generic types can only extend one class. Allow extending multiple classes to enhance flexibility.

Example syntax:

class User<T extends int & String> {
  final T id;
  // Implementation
}

// Or

class User<T extends int | String> {
  final T id;
  // Implementation
}

[mateusfccp]

Duplicate of #2709.