[csswg-drafts] Rename `scrollbar-width` to `scrollbar-thickness`

Zhang-Junzhi has just created a new issue for https://github.com/w3c/csswg-drafts:

== Rename `scrollbar-width` to `scrollbar-thickness` ==
https://drafts.csswg.org/css-scrollbars-1/#scrollbar-width

As a scrollbar width only sounds like the thickness of a vertical scrollbar. For a horizontal scrollbar, the meaning of width doesn't seem very clear, the bar thickness is more likely called height of a horizontal scrollbar.

Thickness is just a direction-neutual word. So what about renaming `scrollbar-width` to `scrollbar-thickness`?

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/2966 using your GitHub account