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/DOM/Document.h

@@ -545,6 +545,8 @@ private:
 
     void tear_down_layout_tree();
 
+    void run_unloading_cleanup_steps();
+
     void evaluate_media_rules();
 
     WebIDL::ExceptionOr<void> run_the_document_write_steps(StringView);