refactor: moved yaruvariant methods inside the enum

[TheLiux]

Description This pull request addresses code organization concerns by relocating the methods of YaruVariant within the enum itself. This refactor takes advantage of the enhanced enums introduced in Dart 2.17. The primary goal is to centralize YaruVariant related code inside the enum itself.

This was one of the edit proposed in this pull request (#398), which i closed and splitted in more pull request as requested

Here is a brief video to show that the variant switch correctly https://github.com/ubuntu/yaru.dart/assets/62995808/ee095f7d-10f7-4414-9396-1c907b676d00