[css] Add support for CSS @supports selector() function

The **CSS Conditional Rules Module Level 4** adds [the `selector()` function to the `@supports` at‑rule](https://drafts.csswg.org/css-conditional-4/#at-supports-ext), which is used to test whether the user agent [supports a new **CSS Selectors** feature](https://drafts.csswg.org/css-conditional-4/#support-definition-ext) (e.g. the `:is(…)` pseudo‑class or multi‑value `:not(…)`). **Visual Studio Code** should provide syntax highlighting and auto‑completion for this.