Thanks for the PR, much appreciated!

I have a couple of questions:

1. For something like:

   @JS("__Swift.Foundation")
   class UUID {
    @JS("__Swift.Foundation")
    func uuidString() -> String {
    Foundation.UUID().uuidString
    }
   }

   Do we actually need the namespace attribute on uuidString here? It seems that namespace on method decl is currently ignored. For such cases where namespace has no effect, it might be less confusing to emit an error or warning. In other words, how about restricting namespace parameter only for top-level declarations?

2. About the public macro JS(_ namespace: String) interface:
   To keep it simple but extensible, how about changing it to public macro JS(namespace: String? = nil)?. Using a labeled parameter leaves room for adding more String parameters in the future. Also since macros can have default arguments, we probably don't need to have a dedicated overload. This would allow us to document both @JS and @JS(namespace:) together in a single DocC page.

Thanks!