Handle variable colors that have variable fallback values

[thecrypticace]

We couldn't parse variables in arbitrary values which had a fallback that was itself a variable and as a result would produce nothing when such a color was used in bg-* or text-* or any other color utility.

Basically this didn't work because the presence of the alpha value forces us to parse the color format:

bg-[rgb(var(--some-var,var(--some-other-var)))]/50

This PR fixes it and it now produces the following CSS:

.bg-\[rgb\(var\(--some-var\,var\(--some-other-var\)\)\)\]\/50 {
  background-color: rgb(var(--some-var, var(--some-other-var)) / 0.5);
}

This implementation is limited because regexes are not recursive. For example, it can't handle another variable fallback like this:

bg-[rgb(var(--some-var,var(--some-other-var,var(--some-other-var-3))))]/50

Doing so would require replacing these regexes with an actual mini tokenizer and parser but that's a task for another day.

Fixes #12048