XHR monkeypatch bridge for transferring Bytes between Elm and JS
without JSON encoding. Consumers use routeUrl to build URLs for
Http.request calls that are intercepted by the companion JS module.
Build a route URL for the XHR bridge.
routeUrl "ws-send" "ws://example.com/chat"
--> "https://xbb.localhost/.xhrhook/ws-send/ws%3A%2F%2Fexample.com%2Fchat"
The URL prefix used by the XHR monkeypatch to intercept requests.