[css-borders-4] Add logical variants of `border-clip-*` longhands

[SebastianZ]

CSS Borders 4 only defines physical longhands for the border-clip property.

To take the writing mode into consideration, the four logical variants border-clip-block-start, border-clip-block-end, border-clip-inline-start, and border-clip-inline-end should be added.

Depending on the outcome of #9252, those might be called border-block-start-clip, border-block-end-clip, border-inline-start-clip, and border-inline-end-clip instead.

Sebastian

[SebastianZ]

A decision on that should be uncontroversial, so adding this to the agenda.

Sebastian

[fantasai]

I'm just going to mark this as accepted Editor Discretion. This feature was only partially drafted in the first place, and this is an obvious fix to that editor's draft. The whole feature needs CSSWG review once it's properly drafted anyway.