Skip escaped commas when splitting selector

[bradlc]

Fixes #5006

This PR ensures that escaped commas within selectors are not considered the end of a selector and the start of a new one.

Currently if you have a prefix (e.g. tw-) a class like dark:tw-bg-[rgb(255,0,0)] can end up generating CSS like this:

.dark .dark\:tw-bg-\[rgb\(255\\,, {
  --tw-text-opacity: 1;
  background-color: rgba(255, 0, 0, var(--tw-bg-opacity));
}

[adamwathan]

❤️