Support view() notation for animation-timeline:
`<view()> = view( [ <axis> || <'view-timeline-inset'> ]? )`
We move AnimationTimeline and its related types into the generics folder,
and define two new structs for scroll() and view().
Note:
- The syntax of scroll() doesn't match the current version of the spec. I will update it in Bug 1814444.
- We will handle the creation/usage of the Anonymous View Progress Timelines in the next patch.