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/Event.h

@@ -29,7 +29,7 @@
 #include <AK/FlyString.h>
 #include <LibWeb/Bindings/Wrappable.h>
 
-namespace Web {
+namespace Web::DOM {
 
 class Event
     : public RefCounted<Event>