Replace `LayerHitNotifier` alias with a type extension

[JaffaKetchup]

What do you want implemented?

Using a type extension instead of a typedef is a good quality-of-life improvement, although breaking.

Here's what it could look like:

extension type LayerHitNotifier<R extends Object>._(
        ValueNotifier<LayerHitResult<R>?> notifier)
    implements ValueNotifier<LayerHitResult<R>?> {
  LayerHitNotifier() : this._(ValueNotifier(null));

  @internal
  @redeclare
  set value(LayerHitResult<R>? v) => notifier.value = v;
}

Creating a notifier becomes more self explanatory:

- final LayerHitNotifier<Object> hitNotifier = ValueNotifier(null);
+ final hitNotifier = LayerHitNotifier<Object>();

Trying to externally set its value (which is not necessary) can be discouraged by the @internal annotation which causes a linting warning.

What other alternatives are available?

No response

Can you provide any other information?

No response