[web-animations] New polyfill

I wrote a polyfill for the spec.

Implemented AnimationTimeline, Animation, KeyframeEffect (only its AnimationEffectpart) according to the spec algorithms. All other code borrowed from web-animations-next.

The main difference with web-animations-next is that it implements the new APIs, and animation timing is live.

https://github.com/hgl/animatable <https://github.com/hgl/animatable>

The only part missing is the pending task rescheduling, as I don’t quite understand what it means and asked in a previous mail. Would like to be enlightened.

Thanks.