[css-snappoints] [css-scroll-snap] Initial merge of the scroll snapping proposals

Hi all,

I've pushed the first step of merging the scroll snapping proposals to the usual place [1].  This includes the core properties from Tab/Elika's proposal for specifying the snap positions from the box rather than a coordinate-based system (adding scroll-snap-padding, scroll-snap-area, scroll-snap-align, removing scroll-snap-destination, scroll-snap-coordinate).

There's obviously still a lot more work to be done, but please do let me know if you see any issues with this core functionality :)

Thanks,
-Matt

[1] https://drafts.csswg.org/css-snappoints/