Don't remove keyframe stops when using important utilities

[thecrypticace]

Fixes #12623

Before:

@keyframes pulse {
  {
    opacity: .5 !important;
  }
}

.\!animate-pulse {
  animation: pulse 2s cubic-bezier(0.4, 0, 0.6, 1) infinite !important;
}

After:

@keyframes pulse {
  50% {
    opacity: .5;
  }
}

.\!animate-pulse {
  animation: pulse 2s cubic-bezier(0.4, 0, 0.6, 1) infinite !important;
}

There's two differences here:

1. We no longer replace the keyframe stops with an empty string — this snuck through because the assumption that all rules are "style" rules is incorrect. This meant that we were looking for classes in this rule when there can never be any.
2. We no longer mark declarations of keyframes as important — browsers ignore these declarations.