Postponed by #3113649: Remove drupal.tabbingmanager's jQueryUI dependency
Problem/Motivation
The :tabbable selector is part of jQuery UI, which has reached end of life. A shim for this selector was added in #3113649: Remove drupal.tabbingmanager's jQueryUI dependency. Now that this is available, we should deprecate :tabbable and refactor all existing uses to use methods from the newly added Tabbable library (and here's where I acknowledge it's confusing to replace the :tabbableselector with methods from a new library called Tabbable).