Animation Worklet Spec Update for Paris F2F

Hi everyone,

In Tokyo we came up with several ideas to address concerns and align the
AnimationWorklet spec with WebAnimations. Since then we have been working
with Brian Birtles and Ian Kilpatrick on the remaining issues and have
updated the spec <https://wicg.github.io/animation-worklet/> and explainer
<https://github.com/WICG/animation-worklet/blob/gh-pages/README.md> to
incorporate these ideas.

We have removed the CSS notation in favor of a clean easily polyfilled
<https://github.com/GoogleChrome/houdini-samples/tree/master/animation-worklet>
Javascript API which is an extension of Web Animations. A WorkletAnimation
is an Animation which runs script to set the localTime on its effect(s)
programmatically in response its input timeline, optionally also using
local state and other timelines.

We believe this will fit in as a custom WebAnimation capable of providing
richer animations while maintaining the performance benefits of running
asynchronously. I'm looking forward to presenting our progress and taking
feedback next week in Paris!

Thanks,
Rob