add support for <timeline-range-name> in @keyframes

[romainmenke]

see : https://www.w3.org/TR/scroll-animations-1/#named-range-keyframes

With scroll driven animations, @keyframes rules can now look like this :

@keyframes {
  cover 50% {
    color: purple;
  }
}

<keyframe-selector> = from | to | <percentage [0,100]> | <timeline-range-name> <percentage>

[aeschli]

Thanks a lot!