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:
1. The syntax of scroll() doesn't match the current version of the spec.
I will update it in Bug 1814444.
2. We will handle the creation/usage of the Anonymous View Progress Timelines
in the next patch.
Differential Revision: https://phabricator.services.mozilla.com/D173904
Although we store animation and transition style values in StyleUIReset and
define their properties in longhands/ui.mako.rs, but we may move them in
the future if this style struct becomes too large. So let's move the
definition of their values to an independent module, animation, so we
don't have to worry about this again.
This patch doesn't change any other things. Only move code.
Differential Revision: https://phabricator.services.mozilla.com/D173903
