[7.x] Add existsOr and doesntExistOr to the query builder

[SjorsO]

In my controllers, I often use something like this to validate requests:

$hasOpenDossier = $user->dossiers()
    ->whereNull('closed_at')
    ->exists();

if ($hasOpenDossier) {
    abort(422, 'User already has an open dossier');
}

With this PR, I can do this instead:

$user->dossiers()
    ->whereNull('closed_at')
    ->doesntExistOr(function () {
        abort(422, 'User already has an open dossier');
    });

This way I don't have to create a temporary variable, and the code is more logically grouped together (which is especially nice in more complex projects where you can have three of these checks in a row).

I would almost always use this feature for aborting. It could also be used for other things, but to be honest I can't really think of any. (maybe adding a doesntExistOrAbort macro to my projects would be a better solution).

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An alternative to adding two new methods is adding an optional $callback argument to the exists() and doesntExist() methods. I think adding two new methods is a cleaner way of doing it, mainly because it keeps the parameters and return type of exists() and doesntExist() simple.

[taylorotwell]

You can send this to 6.x branch since it is not breaking. Thanks.