[css-animations-2][web-animations-2] API For specifying phase linked offsets in keyframes #7637

In #7044 we resolved to add phase linked offsets to the animations APIs. We discussed a simple CSS syntax such as the following:

@keyframes fade-in-out-animation {
  enter 0% { opacity: 0; }
  enter 100% { opacity: 1; }
  exit 0% { opacity: 1; }
  exit 100% { opacity: 0; }
}

We didn't talk about a syntax for the web-animations API, however #7589 touches on an API for the similar delay and endDelay properties so we should probably do the same here. Concretely this means whichever format we choose for #7589 (i.e. String or Object), that could be specified for offset in the Keyframe dictionary and should likely be returned by getKeyframes.