We collapse adjacent rules and media queries automatically. This is because of the way @apply works by generating a Rule/AtRule for every class in an apply. However, when given something like @apply md:text-black md:bg-white we would not collapse the rules inside the media query. This isn't strictly necessary but it would result in a bit of a win with smaller generated CSS if you're using @apply.

I refactored a bit to collapse adjacent rules at the top level and then find all at rules and collapse their children recursively until there's nothing left to collapse.

Fixes #4896