[csswg-drafts] [scroll-animations-1] [css-animations-2] Add animation-timeline (#5159)

majido has just created a new issue for https://github.com/w3c/csswg-drafts:

== [scroll-animations-1] [css-animations-2] Add animation-timeline ==
The css syntax for scroll-linked animations introduces the concept of `animation-timeline` property which allows animation to indicated its timeline via a custom-id or a string name.

@scroll-timeline rule can be then used to specify one such timeline. In future a other at-rules may be introduced for other specific timelines as well.

`animation-timeline` seems more appropriate to belong to css-animations-2 specification. We should move it there. 

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5159 using your GitHub account