feat: Add kanji main meaning

[Adaendra]

As a developer, I want to have a main meaning for each kanji if possible, to facilitate quiz management for this kind of resources.

Requirements

• Add a field main_meaning in the Kanji class

What needs to be done

• Add in the Freezed class a field main_meaning in the Kanji class which can be empty.

Dependencies

Depends on:

• DB #12
• API #75