adopt json_serializable_mobx

[subzero911]

There's a great package https://pub.dev/packages/json_serializable_mobx

What it does:

• serialize ObservableList<T> / ObservableMap<T> / ObservableSet<T>
  (so you don't have to write these awkward converters like ObservableTodoListConverter)
• serialize Observable<T>

Unfortunately, it's abandoned. The only maintainer doesn't support it anymore jperezr21/json_serializable_immutable_collections#28

I ask you to make this code the part of mobx_codegen

[pavanpodila]

Ya we can definitely take this...I had a quick look at the mobx version and there is not much there. We could bring it in easily (I think). Happy to work with you on the PR and review.

[amondnet]

Work in progress