feat: allow KaiselRoute to be implemented, so data-less route families can be enums

[Mastersam07]

KaiselRoute is an abstract class with a const constructor, so every route must extend it. A sealed family whose variants carry no data is exactly an enum — but a Dart enum cannot extend a class, only implement interfaces and apply mixins.

// what you want
enum SettingsTab implements KaiselRoute { profile, security, about }

// what you must write
sealed class SettingsRoute extends KaiselRoute { const SettingsRoute._(); }
final class Profile extends SettingsRoute { const Profile() : super._(); }
final class Security extends SettingsRoute { const Security() : super._(); }
final class About extends SettingsRoute { const About() : super._(); }

Scale

In one 371-route app, 11 of 41 route families had no data on any variant. Those are pure enums expressed as sealed hierarchies — more code, and a lint (prefer_enum_over_sealed_class in that projects house rules) firing on each, requiring an ignore comment per file with an explanation of why the advice is impossible to follow.

Suggestion

Make KaiselRoute implementable — an abstract interface class, or a mixin providing props / routeName defaults — so enum Tab implements KaiselRoute { ... } is legal.

The pieces look compatible: props and routeName are both instance members with defaults, and enums can override both. The blocker is only the const constructor implied by extends.

Worth noting a workaround that is arguably better anyway, and could be documented either way: carry the enum as a field on a single route class.

final class Tab extends KaiselRoute {
  const Tab(this.value);
  final TabValue value;
  @override
  List<Object?> get props => [value];
}

Exhaustiveness then comes from switching on the enum rather than the sealed type, which is equivalent for the builder.