[css-overflow] Remove prev and next scroll-button-direction

[danielsakhapov]

prev and next <scroll-button-direction> values make ::scroll-button() selectors layout dependent, but the spec doesn't define the time and the way browsers should match it.

I want to discuss if we really need them at all, and then either remove them or better specify what and when to do.
This layout dependency for CSS selectors is a new thing, so, I want to also discuss the complexity vs the usefullness of it.

cc: @tabatkins @flackr @lilles

[flackr]

I think one way we could get rid of the layout dependency is to have prev and next be separate from the other scroll-button pseudo-elements. Then the selector always refers to the "prev" and "next" pseudo, whose direction only needs to be known when the button is activated.

That said, I think we could make these work in a similar way to scroll state queries where we rerun style and layout if the matched pseudo changes from the initially matched one.

[tabatkins]

Hm, so having six buttons - block-axis, inline-axis, and longest-axis? That could work.