Move variant sort callback into PageVariant

With \Drupal\Core\Config\Entity\ConfigEntityBase::sort, core seems to be encouraging sorting logic to for config entities to sit within the config entity itself. We currently do our sorting using Page::variantSortHelper.

We could switch over to this pattern by moving Page::variantSortHelper into PageVariant::sort.

We could potentially even use ConfigEntityBase::sort, but that comes with the following caveats:

• PageVariant::$weight is required to be public (though we could post a core issue to switch to using $a->get('weight') rather than $a->weight).
• ConfigEntityBase::sort uses label as a secondary comparison if weights are equal. This may not be desirable.