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.
To prepare for fully qualified tag names, let's call this local_name.
Note that we still keep an Element::tag_name() around since that's what
the JS bindings end up calling into for the Element.tagName property.
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 :^)
