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/Comment.cpp

@@ -27,7 +27,7 @@
 #include <LibWeb/DOM/Comment.h>
 #include <LibWeb/Layout/LayoutText.h>
 
-namespace Web {
+namespace Web::DOM {
 
 Comment::Comment(Document& document, const String& data)
     : CharacterData(document, NodeType::COMMENT_NODE, data)