dart: replace vnum with enhanced enums

We've been using this package for a while: https://github.com/AmirKamali/Flutter_Vnum
It's no longer being maintained.

We can replace it with enhanced enums introduced in dart 2.17 a couple of years ago.

Sample usage:

https://codewithandrea.com/tips/dart-2.17-enums-with-members/
https://medium.com/huawei-developers/flutter-enhanced-enums-the-powerful-feature-of-dart-2-17-new-feature-of-flutter-3-0-8177a926e487