IDL Gen: Support Namespaces

[nikeokoronkwo]

This issue is to add support in the (new) IDL generator for WebIDL namespaces.

Currently, namespaces are generated as getters on a type with properties and methods as shown:

@JS()
external $MyNamespace get MyNamespace

extension type $MyNamespace._(JSObject _) implements JSObject {
  // namespace code...
}

With the refactor, namespace generation will behave more like how it will be in the TS Declaration generator, which will include just having the extension type, and making properties and methods static on the namespace.

[srujzs]

I believe namespaces are always exported through the global context so this transformation should be okay, but something to watch out for if not.