[scroll-animations-1] Make scroll-timeline-* support multiple scroll timelines through list value

[flackr]

The view-timeline rule supports a comma separated list of timelines to declare, however scroll-timeline does not. We should probably support a list value for scroll-timeline as well to allow subsequent siblings (per our resolution on scoped timelines) to animate w.r.t. both the horizontal and vertical scroll positions. e.g.

<style>
.scroller {
  scroll-timeline: scroll-block block, scroll-inline inline;
}
.effect1 {
  animation: effect1 scroll-block;
}
.effect2 {
  animation: effect2 scroll-inline;
}
</style>
<div class="scroller">
  <!-- scrolling content -->
</div>
<div class="effect1"></div>
<div class="effect2"></div>

Note that view-timeline already accepts a list, so this would add more internal consistency.