[7.x] Add 'loadMorphCount' method

[gdebrauwer]

The loadMorph method was added way back in v5.6 by PR #23626.

Because I created a PR for a morphWithCount method, I thought it would be a good idea to add an appropriate load-method.

$comments = Comment::all();

$comments->loadMorphCount('commentable', [
    Post::class => ['likes'],
]);

I also added a proxy method to the paginator, as is the case with loadMorph-method.