[selectors] Make sure :is()/:not()/etc work well with pseudo-elements

[tabatkins]

In the past I've resisted things like .foo:is(.bar, ::before), because the ::before part is technically changing the subject of the compound selector, something no other selector can do.

But this just leaves a functionality gap, since pseudo-elements don't have a combinator dedicated to them to allow matching plainly. Selector syntax is awkwardly designed around pseudo-elements, but we're stuck with it, and should accept that pseudo-elements are part of the compound selector grammar and are allowed to change the subject in a real way.

Thus we should make sure that :not()/etc work properly with them too.

(There's no reason to use :not() with any of the current pseudos, but ::part(foo):not(::part(bar)) makes sense and is useful.)

[Loirooriol]

#2284 resolved to defer this to L5

[tabatkins]

Ah, thanks, this is a dupe then. (I'll copy over my message.)