Further refactor Popover JS module and improve a11y

[sebnitu]

What changed?

This PR refactors some of the core architecture of the popover JS module and adds some handy new features along with some accessibility enhancements. Popovers now require aria-controls="unique-id" and id="unique-id" attributes for triggers and targets respectively. This removes the need to add additional data attributes for popovers and their triggers.

Another big addition in this PR is the Collection class which Popover extends. This allows for a re-usable pattern to build and interface with collections. Lays the groundwork in collections for modal and drawer modules (#835).

Other changes include:

• open() and close() methods now take popover IDs.
• register() and deregister() methods now take popover ID or either popover elements.
• Deprecate closeAll() in favor of calling close() without a parameter to close all popovers.
• Each popover object now has built in methods: open(), close() and deregister().
• Deregister has been refactored to ensure event listeners and object properties are removed.
• New get() method for retrieving collection entries by any popover object property (defaults to id).
• Deprecate the need for data attributes to pass override settings in favor of CSS variables.

Todos

• Introduce new collections class
• Add methods to popover objects
• Add ability to pass popover ID to open, close, register and deregister APIs
• Update tests and add missing coverage
• Update documentation with new APIs

This PR fixes #834