[Suggestion] Rework Enum management

Take this example :

```dart
import 'package:freezed_annotation/freezed_annotation.dart';

part 'example.g.dart';

enum ExampleType { one, two, three }

@JsonSerializable()
class Example {
final ExampleType type;
final String name;

Example({
required this.type,
required this.name,
});

factory Example.fromJson(Map json) => _$ExampleFromJson(json);

Map toJson() => _$ExampleToJson(this);
}
```

Once generated we got :

```dart
// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'example.dart';

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

Example _$ExampleFromJson(Map json) => Example(
type: $enumDecode(_$ExampleTypeEnumMap, json['type']),
name: json['name'] as String,
);

Map _$ExampleToJson(Example instance) => {
'type': _$ExampleTypeEnumMap[instance.type]!,
'name': instance.name,
};

const _$ExampleTypeEnumMap = {
ExampleType.one: 'one',
ExampleType.two: 'two',
ExampleType.three: 'three',
};
```

Json_serializable generates a hard-coded enum ↔ string map in the .g.dart file (e.g. _$ExampleTypeEnumMap). This duplicates information that already exists on the enum itself (Enum.values / Enum.name).

This can be error-prone, especially when the enum comes from an external package (ex: pub.dev, monorepo, ...). If that package updates the enum and build_runner is not re-run, the generated map becomes out of sync with the actual enum definition, potentially leading to errors.

Since enums already expose their values and names, it seems possible to rely on them instead of generating and maintaining a separate hard-coded map.