LibWeb: Add cached global attribute name FlyStrings

Instead of creating extremely common FlyStrings like "id" and "class"
on demand every time they are needed, we now have AttributeNames.h,
which provides Web::HTML::AttributeNames::{id,class_}

This avoids a bunch of string allocations during selector matching.

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -25,6 +25,7 @@
  */
 
 #include <LibWeb/CSS/SelectorEngine.h>
+#include <LibWeb/DOM/AttributeNames.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/DOM/Text.h>
@@ -98,7 +99,7 @@ bool matches(const Selector::SimpleSelector& component, const Element& element)
     case Selector::SimpleSelector::Type::Universal:
         return true;
     case Selector::SimpleSelector::Type::Id:
-        return component.value == element.attribute("id");
+        return component.value == element.attribute(HTML::AttributeNames::id);
     case Selector::SimpleSelector::Type::Class:
         return element.has_class(component.value);
     case Selector::SimpleSelector::Type::TagName: