Resolve #3286466 "Fix accessibility issue"

https://www.drupal.org/project/admin_toolbar/issues/3286466

Improve admin_toolbar accessibility with a better keyboard navigation:

• Major refactoring of the admin_toolbar CSS and JS files.
• Added setting to disable keyboard accessibility features.

Follow-up to MR !58, based on the work of @camilledavis:

• Rebased merge request into a new feature branch.
• Fixed validation errors.