LibWeb: Add XMLHttpRequest Document response type

Author: https://github.com/bplaat
Commit: 222cc29c5c
Pull-request: https://github.com/SerenityOS/serenity/pull/21035
Reviewed-by: https://github.com/kalenikaliaksandr ✅
Reviewed-by: https://github.com/kemzeb

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.idl

@@ -29,6 +29,7 @@ interface XMLHttpRequest : XMLHttpRequestEventTarget {
     readonly attribute unsigned short status;
     readonly attribute ByteString statusText;
     readonly attribute DOMString responseText;
+    readonly attribute Document? responseXML;
     readonly attribute any response;
     attribute XMLHttpRequestResponseType responseType;
     attribute unsigned long timeout;