Add web-animations.js

We will build our SVG SMIL animation implementation on top of the
original Web Animations Polyfill.

The only current difference between this implementation and
https://github.com/web-animations/ is that we have 'transform' declared
as an SVG attribute, using change
https://github.com/web-animations/web-animations-js/pull/606
(which was reverted on the main repository as it affects CSS animations
that use the transform attribute).