LibWeb: Implement ParentNode.append

Author: https://github.com/Lubrsi
Commit: 34dfdc3f37
Pull-request: https://github.com/SerenityOS/serenity/pull/12208
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -66,6 +66,7 @@ interface Document : Node {
     readonly attribute unsigned long childElementCount;
 
     [CEReactions, Unscopable] undefined prepend((Node or DOMString)... nodes);
+    [CEReactions, Unscopable] undefined append((Node or DOMString)... nodes);
 
     Element? querySelector(DOMString selectors);
     [NewObject] NodeList querySelectorAll(DOMString selectors);