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

Author: https://github.com/linusg
Commit: 56550b6ec0
Pull-request: https://github.com/SerenityOS/serenity/pull/17752
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/Window.idl

@@ -18,6 +18,7 @@ interface Window : EventTarget {
     attribute DOMString name;
     [PutForwards=href, LegacyUnforgeable] readonly attribute Location location;
     readonly attribute History history;
+    undefined focus();
 
     // other browsing contexts
     [Replaceable] readonly attribute WindowProxy frames;