feat(GlDisclosureDropdown): Add `GlDisclosureDropdown` component

What does this MR do?

Adds new dropdown component GlDisclosureDropdown

All the use cases should be reflected in the latest Review App stories.

This implementation is heavily based on the previously implemented Listbox component.

Disclosure supports rendering items and groups from config as well as providing content to the default disclosure slot. Item, group header as well as dropdown toggle can be all made custom providing the template to the appropriate slot.

Disclosure in Pajamas

Figma dropdown designs ⚠ disclosure, as well as listbox, still use old styling. This is out of scope here. The redesign of dropdowns will be done separately.

This work is also a preparation for the Navigation Redesign. That's why in the stories I've tried to implement the cases from the new designs - Merge requests dropdown, New Item dropdown, User dropdown. Those should be the most complicated cases though. In most cases, the disclosure will be a simple list of links and actions.

Closes #1632 (closed)