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/CMakeLists.txt

@@ -25,6 +25,7 @@ set(SOURCES
     CSS/StyleRule.cpp
     CSS/StyleSheet.cpp
     CSS/StyleValue.cpp
+    DOM/AttributeNames.cpp
     DOM/CanvasRenderingContext2D.cpp
     DOM/CharacterData.cpp
     DOM/Comment.cpp