Doesn't take into account escaped colons when parsing for :not

[adamwathan]

If I have a class like:

.sm\:not-italic {
  font-style: normal;
}

...this plugin will error when building because the :not in the class name is incorrectly identified as the :not(...) pseudo-class even though the preceding colon is properly escaped.

Happy to dive in and try and fix but if someone more familiar with the project knows exactly how to fix it already I would be extremely grateful for your help! ❤️