CSS snappoints update

People at Mozilla would like to make some progress on a spec, because we'd
like to ship something.

A couple of things have changed since the last go-around:
1) It's become clear that the previous model we had where snap-points
depended, in part, on the direction of a scroll gesture, was a mistake. So
that brings us closer to consensus.
2) Some use-cases which some people wanted CSS snappoints to solve (e.g.
scripted smooth scrolling by a whole page) can now be satisfactorily solved
using CSSOM APIs, e.g. element.scrollBy({ left:..., behavior:"smooth").

I still think a case has not been made for scroll-snap-points-x/y to
support any value other than "elements", so we should get rid of it.

I think scroll-snap-coordinate should allow specifying the type of box used
(margin or border-box, at least) as well as an offset from the edge of that
box.

I still think that scroll-snap-coordinate and scroll-snap-destination are
terrible names.

I'm worried that there are loads of outstanding issues noted in the draft
but there have been no edits since March :-(.

Rob
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