This PR fixes issues #15 and #54. The core idea is to changeup how we style focus, moving from adding the focus-ring class in response to focus events preceded by keydown events, to simply leveraging keydown instead.

As noted in the code comments: keyup provides two advantages over our current approach:

• It will reliably fire when the user is navigating from the browser chrome into the document (fixing issue When to match :focus-ring:? #33).
• Requires one less event listener, while still ensuring we add focus-ring only in response to focus originating from the keyboard.