The scroll snap spec defines the concepts [1]. There are three type of scroll
operations. 1) intended end position, 2) intended direction and end position
and 3) intended direction.

Basically our existing ScrollUnits types correspond;

1) DEVICE_PIXELS, WHOLE => intended end position
2) PAGES => intended direction and end position
3) LINES => intended direction

There are two exceptions in the `intended direction and end position` case,
scrollBy() and fling gestures (on Linux). They were defined as scroll operations
with DEVICE_PIXELS unit, but the spec cleary says they are `intended direction
and end position` operations.

Note that we will also use this information for scroll-snap-stop property since
the properly will only have effects on both 2) and 3) cases.

[1] https://drafts.csswg.org/css-scroll-snap/#scroll-types

Depends on D145190

Differential Revision: https://phabricator.services.mozilla.com/D145191