Shorthand resolve does not work for complex properties like background and border-radius

[Sharktheone]

The problem is in our shorthand resolver here: https://github.com/gosub-io/gosub-engine/blob/main/crates/gosub_css3/src/matcher/shorthands.rs

border-radius has the problem that it's syntax looks like this:

<length-percentage [0,∞]>{1,4} [ / <length-percentage [0,∞]>{1,4} ]?

We first have one to four lengths, this is fine. But then optionally it has a / followed by again one to four lengths. The current logic would know how to resolve the first part, for the second part it is a bit more difficult, since it needs to be again resolved to the same properties as the first one.