Generate `copyWith` etc without generating `fromJson` etc

[caseycrogers]

I mostly use dart mappable for classes that I want to (de)serialize. However, sometimes, I just need copyWIth etc for a class that I don't intend to serialize/deserialize.

I can either user dart_mappable and ignore the generated serialization related methods (not ideal because the class may not even be coherently serializable eg if it needs custom mappers for some of its attributes) or use another package (also not ideal because now I have two different packages with different syntax for doing roughly the same thing).

Ideally, I could just specify a false bool flag in the annotation that would tell dart mappable to skip generating serialization related methods.

[schultek]

I got you: https://pub.dev/documentation/dart_mappable/latest/topics/Configuration-topic.html#generation-methods