[css-gaps-1] Shorthand for rule-break and rule-outset

[kbabbitt]

@JoshTumath suggested creating a shorthand for rule-break and rule-outset since there's a coupling between them. Currently, there's no intersection in the value space between them, so we could just jam them together like so:

rule-break: intersection;
rule-outset: 0px;
/* shorthands to: */
rule-bikeshedme: intersection 0px;

In typing that out, I realized there's not an obvious name for such a shorthand. rule is already taken as the shorthand for width, style, and color.

There's also a couple of discussions, such as #12024, calling for more flexibility in defining outsets, which might be a further consideration in designing such a shorthand.