Add @variants at-rule for generating multiple variants

[adamwathan]

Adds the ability to generate hover and focus variants of a utility at once:

@variants hover, focus {
  .bg-brand { ... }
}

This will likely replace the @hoverable directive in the next breaking release. We've also had a intentionally undocumented @focusable at-rule for a while, but the issue was combining hoverable and focusable didn't work as expected because you had to nest them:

@hoverable {
  @focusable {
    .bg-brand { ... }
  }
}

This would generate a bunch of insane crap, so this PR fixes that by allowing you to generate them at the same time on the same set of source rules instead of running them on top of each other.

The main purpose of this PR is to lay the groundwork for a config-based approach to specifying which utilities should have hover, focus, and responsive variants. The work there is mostly done, just needs some tweaks after this gets merged.