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.
The more generic virtual variant is renamed to node_name() and now only
Element has tag_name(). This removes a huge amount of String ctor/dtor
churn in selector matching.
