[css-scroll-snap-1] Panning gesture should be treated as relative for last scroll direction.

In css-scroll-snap-1, absolute scrolls were defined to include a panning gesture released without momentum. This would lead to the scroll direction not changing while panning up/down which doesn't make much sense as from a user perspective it is a relative scroll and the user would expect it to show/hide scroll direction based ui while panning.

I think we should include panning with and without momentum as relative scrolls as this makes the most sense for all of the other implications suggested in #12394 and figure out if / how to change the snapping behavior for this special case. E.g. maybe the end of a relative scroll includes a direction hint as to whether the scroll should continue in that direction or target the end location?

@tabatkins @tursunova

Start with the css-scroll-snap-1 definition of absolute scrolls and review the implications discussed in issue #12394. Determine how panning with and without momentum should be classified and how snapping should behave, then update the specification so the chosen direction and end behavior are explicit.