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/PseudoClasses.json

@@ -164,6 +164,9 @@
   "target": {
     "argument": ""
   },
+  "unchecked": {
+    "argument": ""
+  },
   "user-invalid": {
     "argument": ""
   },

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,

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -616,6 +616,8 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         return element.matches_enabled_pseudo_class();
     case CSS::PseudoClass::Checked:
         return element.matches_checked_pseudo_class();
+    case CSS::PseudoClass::Unchecked:
+        return element.matches_unchecked_pseudo_class();
     case CSS::PseudoClass::Indeterminate:
         return matches_indeterminate_pseudo_class(element);
     case CSS::PseudoClass::HighValue: