[css-scroll-snap-2] Expose location of snap points to javascript

[johannesodland]

To make it possible to build custom behavior around scroll-snapping, the position of snap points should be exposed to javascript.

When extending scroll-snap functionality it is sometimes necessary to know the exact location of the snap-points. This is currently necessary when i.e. implementing scrolling to snap point or extending padding to ensure all snap-points are reachable.

Currently, it is possible to compute the location of the snap-points by getting the computed styles of the scroll container and target element, and measuring their dimensions.

With the change in #11165 this might become impossible, as the browser will distribute some amount of the initial and ending scroll to ensure that the scroll position of the unreachable snap points does not overlap.