feat: Implement `KnowledgeLevel`, `Stage`, and `Level` schemas

[apomalyn]

Requirements

Implement using Freezed and Isar the models of KnowledgeLevel (schema), Stage, and Level (schema).

Implement a LearningMode enum with the following elements:

• review
• learn

Update the User model to include the learning section (schema)

What needs to be done

Update the ResourceUid to include Stage and Level uid format

Implement under src/core/models the following models using Freezed and Isar:

• KnowledgeLevel: as it's own collection
• Stage: as it's own collection
• Level: as an embedded

Under src/core/constants, implement LearningMode enum.

Update the User model to have the knowledgeLevels array

Dependencies

Child of #218

Depends on:

• https://github.com/RoadTripMoustache/kana_to_kanji_api/issues/77

• 216

Dev notes

• Freezed
• Isar