LibWeb: Add "tag name" buckets to StyleComputer::RuleCache

We can skip rules that require a specific tag name when matching against
any element with a different tag name. :^)

Userland/Libraries/LibWeb/CSS/StyleComputer.h

@@ -98,6 +98,7 @@ private:
     struct RuleCache {
         HashMap<FlyString, Vector<MatchingRule>> rules_by_id;
         HashMap<FlyString, Vector<MatchingRule>> rules_by_class;
+        HashMap<FlyString, Vector<MatchingRule>> rules_by_tag_name;
         Vector<MatchingRule> other_rules;
         int generation { 0 };
     };