LibWeb: Add namespaces to Attribute selectors

For now, we parse these, but don't actually consider the namespace when
matching them. `DOM::Element` does not (yet) store attribute namespaces
so we can't check what they are.

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -177,7 +177,7 @@ public:
                 CaseInsensitiveMatch,
             };
             MatchType match_type;
-            FlyString name {};
+            QualifiedName qualified_name;
             String value {};
             CaseType case_type;
         };