LibWeb: Implement the :target selector pseudo-class

We don't yet set the Document's target element in most cases, so this
does not function very well. But that will improve once we *do* set it,
which involves a more complete Navigables implementation.

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -123,6 +123,7 @@ public:
                 VolumeLocked,
                 Buffering,
                 Stalled,
+                Target,
             };
             Type type;
 
@@ -344,6 +345,8 @@ constexpr StringView pseudo_class_name(Selector::SimpleSelector::PseudoClass::Ty
         return "buffering"sv;
     case Selector::SimpleSelector::PseudoClass::Type::Stalled:
         return "stalled"sv;
+    case Selector::SimpleSelector::PseudoClass::Type::Target:
+        return "target"sv;
     }
     VERIFY_NOT_REACHED();
 }