refactor(Recipe): rename constructor fromJsonMap to fromJson

- Follow the convention of symmetrically naming to/from JSON converters as illustrated in [Dart Up and Running, Chapter 2 section on "Classes"](https://www.dartlang.org/docs/dart-up-and-running/contents/ch02.html#classes) — viz. showing named constructor `Point.fromJson()`.
- Also using literal constants instead of `new Map` and `new List`.