Support Namespace const in webidl (#30492)

* Add TestNS with const

* Implement namespace const in codegen

components/script/dom/testns.rs

@@ -0,0 +1,7 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// check-tidy: no specs after this line
+
+pub struct TestNS(());