Add MappableBase<TSelf> interface to guarantee toMap, toJson, and copyWith across generated models

[okandemirofficial]

Summary

This PR introduces a small, self-referential generic interface MappableBase<TSelf extends MappableBase>.
All generated model mixins now implement MappableBase, so generic services can rely on toMap(), toJson(), and copyWith without extra flags.

Why

I often write generic helpers/services that should work with any mappable type:

class MyService<T extends MappableBase> {
  void convertToJson(T value) {
    final json = value.toMap();
  }
}

Having a shared capability interface lets the compiler enforce these features at call sites.

What changed

Added base mixin MappableBase

mixin MappableBase<T> {
  String toJson();
  Map<String, dynamic> toMap();

  @override
  String toString();

  @override
  bool operator ==(Object other);

  @override
  int get hashCode;

  ClassCopyWith<T, T, T> get copyWith;
}

Updated the generator to emit:

mixin MyModelMappable implements MappableBase {

If this approach doesn’t make sense or there’s a better way to achieve it, please feel free to let me know and disregard the PR.

I really appreciate your work — thanks in advance for taking the time to review this!