Possibility of adding `scrollbar-button-start` & `scrollbar-button-end` to CSS Scrollbars Styling spec

[Schepp]

The scroll buttons are basically describing what we already get with scroll containers today on desktop devices in regards to the scrollbar and its arrow buttons. Maybe it makes sense to extend the CSS Scrollbars Styling spec with ::scrollbar-button-start and ::scrollbar-button-end, so that we can use what is already there?