Animation Worklet moved to WICG

Based on the discussion at TPAC I've moved our Animation Worklet proposal
<https://discourse.wicg.io/t/proposal-animationworklet-a-primitive-for-scroll-linked-and-high-performance-procedural-animated-effects/1710>
to the WICG GitHub <https://github.com/WICG/animation-worklet> and deleted
<https://github.com/w3c/css-houdini-drafts/commit/c8e915845be1c699052f3f577f8ace1a68576263>
the old "composited-scrolling-and-animation" folder from css-houdini-drafts.

I believe I've captured a summary of the feedback from discussions at TPAC as
issues in the repo <https://github.com/WICG/animation-worklet/issues>,
please add any additional feedback there.  As we discussed at the meeting,
our hope is to scope the problem down sufficiently such that we can
consensus on a simple MVP design which we could conceivably ship in Chrome
relatively soon (eg. in the next 6 months).

Thanks!
   Rick