add support for optimizing the CSS `:has` / `:is` pseudo-selectors

[thescientist13]

Type of Change

Feature

Summary

The CSS :has psuedo-selector is neat new CSS feature (almost*) available in all browsers and can [come in pretty handy]
(https://twitter.com/wesbos/status/1653401308022607874?s=43&t=23JfvPjOIJyZ9W8iFiZYyg).
https://www.smashingmagazine.com/2023/01/level-up-css-skills-has-selector/

/* Selects an h1 heading with a
paragraph element that immediately follows
the h1 and applies the style to h1 */
h1:has(+ p) {
  margin-bottom: 0;
}

It is still outstanding in FF but will be coming soon but it is a standard now, so no harm in supporting it in Greenwood
https://caniuse.com/css-has

Details

So will just need to add support for this in our CSS resource plugin and add a test case.