[css-borders-4] Allow to individual values for sides in `border-clip`

The border-clip shorthand currently sets all four sides to the same value. Though it would make sense to extend its syntax to set different values for the four sides.

To do that, the syntax of grid-area could be reused, i.e.

border-clip = <border-clip-pattern> [ / <border-clip-pattern> ]{0,3}

<border-clip-pattern> = normal | [ <length-percentage [0,∞]> | <flex> ]+

where the number of values works the same as for the box properties like margin. I.e. one specified value applies to all four sides. Are two values specified, the first applies to the top and bottom and the second to the left and right side. With three values specified, the first one applies to the top, the second to the left and right and the third to the bottom side. And are four values specified, they are applied to the top, right, bottom, and left sides in that order.

Sebastian

Start by locating the border-clip definition in the CSS Borders 4 material and compare its syntax with the grid-area pattern referenced in the issue. Confirm how one, two, three, and four values map to sides, then update the specification and any affected examples or checks so the proposed shorthand behavior is documented.