Rework button focus/active styling, with extra changes for checks/radios

This long-overdue PR does a few things concerning buttons:

• for all .btns, it changes the :focus styling to :focus-visible - this way, the focus style doesn't take effect and "stick" (until you click somewhere else) for mouse/touch users
• for button-like checks and radios, the :focus-visible style now doesn't set the background colour, and there is no :hover styling. this stops the very confusing problem of having the mouse, or keyboard focus, on a checkbox, checking/unchecking it, but only being able to see whether they did indeed check/uncheck by moving keyboard focus away/clicking somewhere else. particularly noticeable in our outline button example, where before you simply couldn't tell what was going on.

Closes #33481 (closed), closes #31149 (closed), closes #34664 (closed), closes #36502 (closed), Fixes #26804 (closed).

Possibly supersedes #28999 (think this approach may be cleaner)