LibWeb: Add HTMLOptionsCollection selected index property

Author: https://github.com/bplaat
Commit: 5decf4b33c
Pull-request: https://github.com/SerenityOS/serenity/pull/23895
Reviewed-by: https://github.com/shannonbooth

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.idl

@@ -9,5 +9,5 @@ interface HTMLOptionsCollection : HTMLCollection {
     // [CEReactions] setter undefined (unsigned long index, HTMLOptionElement? option);
     [CEReactions] undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
     // [CEReactions] undefined remove(long index);
-    // attribute long selectedIndex;
+    attribute long selectedIndex;
 };