fix selector order in pseudo :is

[romainmenke]

Pseudo elements ordering appears important : w3c/csswg-drafts#7085 (comment)

With re-ordering in compound selectors we broke this in :is()

---

Before this change :

:is(a, button):is(:hover::before, :focus) {
	order: 25;
}

/* becomes */

button::before:hover {
	order: 26;
}

After this change :

:is(a, button):is(:hover::before, :focus) {
	order: 25;
}

/* becomes */

button:hover::before {
	order: 26;
}

Also added a test in nesting to prevent introducing the same bug in the future and clarified a few comments.

[Antonio-Laguna]

Nice one! Thanks for fixing this!