fix: Use freezed on Isar collections

[apomalyn]

As a developer, I want to reduce the boilerplate code I need to do.

What needs to be done

• Modify the Isar collection class (Group, Kana, Kanji, Vocabulary at the time of writing) to leverage Freezed code generation.

Dev notes

Here is a tutorial, on how to implement that.

[Adaendra]

Actually there is an issue with 2 things :

• enumeration
• String id

Note that isar_generator is included in isar lib since the v4.0.0.