Split JS and CSS loading of the polyfill

[bramus]

Fixes #160.

Not sure if we do want to have this though, as it will leave Firefox Nightly somewhat useless state for CSS-based SDA’s.

With the changes in this PR, the polyfill won’t kick for the CSS part but will for the JS part in Firefox Nightly. However, because Firefox Nightly supports an older CSS syntax (e.g. using vertical instead of y, or requiring an time-based animation-duration) this will leave CSS Scroll-Driven Animations in a broken state.

One thing I do like about this PR is the code restructure that splits off the JS plugging into scroll-timeline-js.js.