LibWeb/HTML: Port Window.confirm() to IDL

Author: https://github.com/linusg
Commit: bbffda5f55
Pull-request: https://github.com/SerenityOS/serenity/pull/17752
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/Window.idl

@@ -7,6 +7,7 @@ interface Window : EventTarget {
     // user prompts
     undefined alert();
     undefined alert(DOMString message);
+    boolean confirm(optional DOMString message = "");
 };
 Window includes GlobalEventHandlers;
 Window includes WindowEventHandlers;