LibWeb: Clear all active timers when document is destroyed

This change implements a step from the document's destroy procedure in
the specification, saying that all active timers should be cleared.

By doing this, we also fix the leaking of a document in case where we
have navigated away from a page that has scheduled timers that haven't
yet been triggered.

Userland/Libraries/LibWeb/HTML/Timer.h

@@ -24,6 +24,7 @@ public:
     virtual ~Timer() override;
 
     void start();
+    void stop();
 
 private:
     Timer(JS::Object& window, i32 milliseconds, Function<void()> callback, i32 id);