Add the proposed syntax stuff from the f2f discussion

[tabatkins]

This relates to the following:

• #12652 animation-trigger CSS Syntax
• #12611 Set of actions for AnimationTrigger
• #12336 Move scroll and event animation triggers to independent namespace
• #12581 animation-trigger name clash resolution
• #12399 Should an animation be allowed to have multiple triggers associated? (Resolution says only one trigger per animation right now, but I went ahead and did multiple with the proposed syntax in the issue, since other convos settled us on a usable syntax for this.)

This does not handle the OM side of things yet, as the point of this is just to give us a single thing to look at when getting final approval, rather than scattered decisions across multiple issues.