[11.x] Add generic PHPDoc to relations

[asanovr]

These changes add support to generic types for model relations.

Now, we can write types like this:

class User extends Model {
    /**
     * @return HasMany<Post>
     */
    public function posts(): HasMany {
        return $this->hasMany(Post::class);
    }
}

And we will get

$user->posts()->get() - Collection of Post $user->posts()->make() - Model of Post

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[siarheipashkevich]

@nunomaduro why didn't you do it earlier? 😄

[browner12]

from a semantic perspective, don't "child" and "related" reference the same thing? Would "child" and "parent" be more appropriate?

[taylorotwell]

We don't really have any extensive type test suit here so it's hard to ship this with confidence.

[calebdw]

Most of this is likely coming from the Larastan stubs (and I've thought of attempting a PR like this several times).

However, since we're looking to add this to the framework now I do want to call attention to @nagmat84's extensive PR to fix the Larastan relation generics: https://github.com/larastan/larastan/pull/1285. The PR was closed when @nunomaduro deleted the Larastan master branch and it just hasn't been reopened, but I believe it would be good to take the proposed changes under consideration.

[calebdw]

@asanovr, @driesvints, this can be closed now that #51851 has been merged

[driesvints]

Thanks @calebdw