Extract keyframe name when followed by comma

[philipp-spiess]

Fixes #17332

This PR ensures that keyframes are emitted even when they are referenced following a comma, e.g.:

@theme {
  --animate-test: 500ms both fade-in, 1000ms linear 500ms spin infinite;
                                 /* ^ */

  @keyframes fade-in {
    from {
      opacity: 0%;
    }
    to {
      opacity: 100%;
    }
  }
}

Test plan

Added a unit test to capture the issue from #17332