Make boot URL available somehow

For most apps, the initial URL of an app is `/`, but when using the `visit()` API, it may be useful to know this information later down the path (ignoring any redirects or other transitions that may happen by the app).

This `bootURL` could be available on the `router` or the router service, but because it can also be useful to see it in an initializer, and neither of those are fully setup during the boot process, some other mechanism may be required.