LibWeb: Add internals call to dump display list

It's useful to have tests that dump display list items, so we can more
easily see how changes to the display list recording process affect the
output. Even the small sample test added in this commit shows that we
currently record an unnecessary AddClipRect item for empty paint phases.

For now, the dump doesn't include every single property of an item, but
we can shape it to include more useful information as we iterate on it.

Libraries/LibWeb/Internals/Internals.cpp

@@ -257,4 +257,9 @@ bool Internals::headless()
     return page().client().is_headless();
 }
 
+String Internals::dump_display_list()
+{
+    return window().associated_document().dump_display_list();
+}
+
 }