Support @supports selector()

[Seirdy]

The @supports at-rule now supports the selector() function as of CSS conditionals level 4.

Safari, Chromium, Edge, and Firefox have shipped support. The CSS validator and Nu validator fail to parse it.

Contrived test-case:

@supports selector(:focus) {
	code {
		display: inherit
	}
}

Real-world test-case: the <style> element for https://seirdy.one/