Modernize and rework input focus states

[tristen]

Looks like this:

Summary of changes

• Native focus states: Focus outlines use the browser's :focus-visible, so keyboard users see them and mouse users don't.
• New outline style: Focused elements get a 2px solid blue outline instead of a soft blue glow.
• Form controls: Checkboxes, radios, switches and toggles get the outline. On range sliders it goes on
  the thumb.
• assembly.js is now optional for focus: It only hides the outline when a select is clicked. Without it, focus states still work everywhere.
• Breaking: The --focus-shadow variable is replaced by --focus-outline.

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Closes #998