do not lint on ThisNotifier build() => this in generated code

[RandalSchwartz]

Describe the bug
riverpod_lint is complaining for notifiers that are of their own type, useful when you want to migrate a changenotifier to not
have a separate state instance. This causes ThisProvider build() => this; to be linted, even though everything works.

To Reproduce

@Riverpod(keepAlive: true)
class RubberDuck extends _$RubberDuck {
  @override
  RubberDuck build() => this;
}

and of course the proper imports and build structure.

Expected behavior

The RubberDuck type, a derived class of Notifier, would be accepted and supported as the return type.

[rrousselGit]

Honestly you probably shouldn't do that though

[RandalSchwartz]

It's the easiest thing possible when porting over a ChangeNotifier to the new framework. There's no specific State type object to work with, and using the Notifier itself shortens the mutate methods. If there's a way to accomplish those easier, I'm listening...

[rrousselGit]

Moving all the properties to a mutable state class is decently simple too.