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/BrowsingContext.h

@@ -96,6 +96,7 @@ public:
     }
 
     bool is_top_level() const;
+    bool is_auxiliary() const { return m_is_auxiliary; }
 
     DOM::Document const* active_document() const;
     DOM::Document* active_document();