fix(recipe): follow new dart:convert.JsonCodec JSON encoding convention

- `recipe.dart` of Chap. 5:
  - Replace `toJsonString()` by `Map toJson()` as documented in
[dart:convert.JsonCodec#encode](https://api.dartlang.org/apidocs/channel
s/stable/#dart-convert.JsonCodec@id_encode)
  - Some code cleanup.
- Chap. 6 & 7 made to match updates to Chap. 5.
Undo formatter line wrap on constructor
Merge pull request #1 from chalin/patch-0305-json

Fix `toJsonString()`