LibWeb: Make CSS rule cache smarter about pseudo elements

Instead of putting every rule that matches a pseudo element in the
same bucket, let them go in the best ID/class/tag name bucket instead.
Then, add a flag to MatchingRule that says whether it contains a
pseudo element in the rightmost compound selector.

When deciding which selectors to run for an element, we can now simply
filter in/out pseudo element selectors as appropriate depending on what
we're trying to match.

This fixes an issue where pages using Font Awesome had 1700+ rules in the
pseudo-element rule cache. (This meant all those rules had to run
against every element twice or more while instantiating pseudo elements.)

Userland/Libraries/LibWeb/CSS/StyleComputer.h

@@ -26,6 +26,7 @@ struct MatchingRule {
     size_t rule_index { 0 };
     size_t selector_index { 0 };
     u32 specificity { 0 };
+    bool contains_pseudo_element { false };
 };
 
 class PropertyDependencyNode : public RefCounted<PropertyDependencyNode> {
@@ -111,7 +112,6 @@ private:
         HashMap<FlyString, Vector<MatchingRule>> rules_by_id;
         HashMap<FlyString, Vector<MatchingRule>> rules_by_class;
         HashMap<FlyString, Vector<MatchingRule>> rules_by_tag_name;
-        HashMap<Selector::PseudoElement, Vector<MatchingRule>> rules_by_pseudo_element;
         Vector<MatchingRule> other_rules;
     };