Install @@toStringTag on namespace objects

A followup to #357.

This change aligns WebIDL namespace objects with ECMA-262 ones, per tc39/ecma262#2057 (comment).

Tests: web-platform-tests/wpt#24717

Author: shvaikalesh

index.bs

@@ -14080,6 +14080,8 @@ The characteristics of a namespace object are described in [[#namespace-object]]
     1.  Return |namespaceObject|.
 </div>
 
+The [=class string=] of a [=namespace object=] is the [=namespace=]’s [=identifier=].
+
 
 <h3 id="es-exceptions">Exceptions</h3>