LibWeb/CSS: Add :unchecked pseudo-class

This just got added to the Selectors spec:

b78c97c19d

It's thus missing from the HTML spec and WPT, but I figured it was
simple enough to add.

Libraries/LibWeb/CSS/Selector.cpp

@@ -62,6 +62,7 @@ static bool can_selector_use_fast_matches(Selector const& selector)
                         PseudoClass::OnlyChild,
                         PseudoClass::Root,
                         PseudoClass::State,
+                        PseudoClass::Unchecked,
                         PseudoClass::Visited))
                     return false;
             } else if (!first_is_one_of(simple_selector.type,