Remove the model property from Factories and add extends annotation

[GeniJaho]

The $model property is not needed anymore starting from Laravel 8 laravel/framework#39310, as the Factory can find the name of the Model to create in other ways.

In Laravel 9, however, they made the factories generic laravel/framework#39169. It allows for much better static analysis and autocompletion in tests.

This PR adds a new rule to the Laravel 8.0 set that removes the $model property from all factories and adds the @extends annotation to the Factory with the removed $model value. Users can only reap the benefits of the annotation starting from Laravel 9.0.

The reason I did not add an extra rule to add the annotation separately is that we don't have a way to correctly determine the name of the model to use. We can add the annotation safely only if we get it from the user-defined $model value.

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Update

Removing the $model property actually works only if the Models are in the App\Models or App namespace, so removing them by default doesn't make much sense. I will make two separate rules in this case:

• Rule to remove the $model property, outside of any set list.
• Rule to add the @extends annotation if there is a $model property defined, which will be part of the Laravel 9 set list.