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.
It's still only a dummy as LibWeb doesn't have focused elements yet, but
at least now we don't treat "selector:focus" as just "selector".
This fixes an issue on google.com which was mostly grey - coming from
some menu item focus styles :^)
