[animation-trigger] Update animation-trigger parsing

The updated plan for animation-trigger related properties is to move
properties specific to types of triggers to separate CSS property
namespaces.

Under this new format, animation-trigger simply specifies a list of dash
idents which are matched against names declared by the type-specific
trigger properties.

Here is an example of how the property will be used:

```CSS
  timeline-trigger: --trigger view() repeat contain 0% contain 100%;
}

  animation: animation1;
  animation-trigger: --trigger;
}
```

```HTML
<div id="scroller">
  <div id="viewtarget1"></div>
  <div>Lots of other content.</div>
  <div id="viewtarget2"></div>
</div>

<div id="animatable"></div>
```

In this example, scrolling either viewtarget into view will play the
animation. More examples can be seen at [2]

The tests in animation-trigger-parsing.tentative.html have been modeled
to follow the precedent set by anchor-name.

[1] w3c/csswg-drafts#12336 [2]
https://github.com/explainers-by-googlers/scroll-triggered-animations/blob/main/CSS_PROPERTIES.md

Change-Id: I532e5c65feee8676ed56b62cd178c6efa13afe81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6717917
Reviewed-by: Daniil Sakhapov <[email protected]>
Reviewed-by: Kevin Ellis <[email protected]>
Commit-Queue: David Awogbemila <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1487150}

Author: David Awogbemila