Eliminate irrelevant rules when applying variants

[thecrypticace]

When adding utilities or components with a top-level at-rule we capture that entire object as the relevant thing for each of the candidates. We then do selector and rule elimination after the fact — especially as it relates to applying variants.

Take the HTML:

<div class="md:w-full"></div> 

And this plugin:

addUtilities({
  '@supports (foo: bar)': {
    // This doesn't contain `w-full` so it should not exist in the output
    '.outer': { color: 'red' },
    '.outer:is(.w-full)': { color: 'green' },
  },
})

With this we would look at .outer and eliminate any selectors that do not contain .w-full — which is all of them. This meant that we would try to output a rule without a selector. Oops.

Instead, we should eliminate any selectors that are not applicable and then, if the list becomes empty, eliminate that rule entirely.

Fixes #12111