Updates to CI and code cleanup

[martijn00]

Solution description

Closes:

• https://github.com/flutter-form-builder-ecosystem/form_builder_validators/issues/73
• https://github.com/flutter-form-builder-ecosystem/form_builder_validators/issues/76
• https://github.com/flutter-form-builder-ecosystem/form_builder_validators/pull/80
• https://github.com/flutter-form-builder-ecosystem/form_builder_validators/pull/77
• https://github.com/flutter-form-builder-ecosystem/form_builder_validators/issues/90

I've used this opportunity to also cleanup some code and CI. Let me know if i need to change anything.

It might be necessary to run flutter gen-l10n as a build step, but i can validate that once you approve running this PR.

Screenshots or Videos

To Do

• [ ] Read contributing guide
• [ ] Check the original issue to confirm it is fully satisfied
• [ ] Add solution description to help guide reviewers
• [ ] Add unit test to verify new or fixed behaviour
• [ ] If apply, add documentation to code properties and package readme