LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
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.
