[selectors-4] Rename :focus-ring to :focus-visible

[alice]

Per discussion in #2036

[frivoal]

Looks good to me.

Just one little non essential comment:
This has been up under the focus-ring name for a while, so there could be quite a few inbound links. Just in case, it could be nice to keep a (hidden) element with the old ids (the-focusring-pseudo, focus-ring-pseudo) near the new ones, to keep the links working.

[alice]

Good idea, will fix.

[tabatkins]

This is done by adding oldids="whatever" to the heading element, btw. No need to manually add an element.

[alice]

Done.

[alice]

I can't pull this but seems like it's ready to go?