LibWeb: Move DOM classes into the Web::DOM namespace

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.

Libraries/LibWeb/DOM/Node.cpp

@@ -48,7 +48,7 @@
 
 //#define EVENT_DEBUG
 
-namespace Web {
+namespace Web::DOM {
 
 Node::Node(Document& document, NodeType type)
     : m_document(&document)