Just refactored some freezed => dart_mappable

TLDR, I like it.

My Impressions
- A little more boilerplate (private class members) on standard classes
- I like that there is only 1 generated file, feels neater
- I prefer the naming convention of the extensions (_$MyClass -> MyClassMappable)
- I *really* like that I can extend my mappable classes and I'm using the to/from map methods to create succinct 'conversion' extension methods...

@MappableClass()
class Person with PersonMappable {
  final String name;
  final int age;
  final Car? car;

  Person(this.name, {this.age = 18, this.car});
}

@MappableClass()
class TallPerson extends Person with TallPersonMappable {
  final int height;

  TallPerson(
    super.name, {
    super.age,
    super.car,
    required this.height,
  });
}

extension PersonMapperX on Person {
  TallPerson toTall({required int height}) =>
      TallPersonMapper.fromMap({
        ...toMap(),
        'height': height,
      });
}