scroll-policy: ease horizontal scrolling by wheel in vertical writing mode

I propose to introduce a new property scroll-policy, in order to ease horizontal scrolling in vertical writing mode:

The property has two values: none (default value), auto.

none keeps the traditional scroll policy unchanged, in order to avoid breaking upward compatibility.

auto tells the browsers to take full advantage of scrolling operations, as much as they like, without fearing any compatibility:

For example, if an element is in vertical-rl mode, and overflows in only the horizontal direction, then the browser can quite confidently set the default action of the upward wheel event as rightward scrolling, and the default action of the downward wheel event as leftward scrolling.

This feature is significantly helpful if the whole page is more than one screen in vertical writing mode, and will make the vertical writing pages eventually become practical in real-world.

Start with issue #4380 and its 14-comment discussion; review the proposed scroll-policy values, vertical writing mode, and wheel-event behavior. No files, tests, or implementation entry points are named in the payload; done requires an agreed specification for the property and its scrolling semantics.