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