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/Timer.cpp

@@ -30,7 +30,7 @@
 #include <LibWeb/DOM/Timer.h>
 #include <LibWeb/DOM/Window.h>
 
-namespace Web {
+namespace Web::DOM {
 
 NonnullRefPtr<Timer> Timer::create_interval(Window& window, int milliseconds, JS::Function& callback)
 {