AK: Rename downcast<T> => verify_cast<T>

This makes it much clearer what this cast actually does: it will
VERIFY that the thing we're casting is a T (using is<T>()).

Userland/Applications/SystemMonitor/ThreadStackWidget.cpp

@@ -74,7 +74,7 @@ void ThreadStackWidget::refresh()
 
 void ThreadStackWidget::custom_event(Core::CustomEvent& event)
 {
-    auto& completion_event = downcast<CompletionEvent>(event);
+    auto& completion_event = verify_cast<CompletionEvent>(event);
 
     StringBuilder builder;