Fix @apply of animation utilities

[thecrypticace]

Fixes #94

[RobinMalfait]

Hey! Thank you for your PR!
Much appreciated! 🙏

It's looking very promising, just found a little bug!
If you like, I can update the PR for you, but just wanted to let you know about what I found.

[RobinMalfait]

This is looking good, however I think we can improve this a little bit.
Inside keyframes, you are not allowed to have !important, this is invalid:

 @keyframes spin {
   to {
     transform: rotate(360deg) !important;
   }
 }

But since the if (canRewriteSelector) is only wrapping the rule.selector = ... change, this walkDecls to change the important will still happen.

We can fix that, and we can also skip the whole root.walkRules I think:

if (canRewriteSelector) {
  root.walkRules((rule) => {
    rule.selector = replaceSelector(
      apply.parent.selector,
      rule.selector,
      applyCandidate
    )

    rule.walkDecls((d) => {
      d.important = important
    })
  })
}

You can see this behaviour when you do this:

.foo {
  @apply animate-spin !important;
}

[thecrypticace]

You know I thought about this — I wasn't sure if that was invalid or not so I left it just in case. I've updated the implementation.

[RobinMalfait]

Perfect, thanks!