LibWeb: Implement window.close and window.closed

Author: https://github.com/trflynn89
Commit: aa1df95b31
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1644
Reviewed-by: https://github.com/shannonbooth ✅

Userland/Libraries/LibWeb/HTML/Navigable.h

@@ -69,6 +69,7 @@ public:
 
     bool is_closing() const { return m_closing; }
     void set_closing(bool value) { m_closing = value; }
+    bool is_script_closable();
 
     void set_delaying_load_events(bool value);
     bool is_delaying_load_events() const { return m_delaying_the_load_event.has_value(); }