add a GoRouter.builder ctor

[csells]

it should take two functions:

1. a navigate function that does the redirection and builds up the stack of page builders
2. a builder function that creates the Navigator with the stack of pages from the page builders

GoRouter.builder(
  String Function(String location) navigate, // returns whatever loc redirection lands on
  Widget Function(BuildContext context, String location) builder, // builds the Navigator
)

// the delegate implementation becomes easy now
void go(String location) {
  _loc = this.navigate(location);
  notifyListeners(); // calls build()
}

Widget build(BuildContext context) => this.builder(context);

That's what the real API is for Nav2 and on top of that can be built the declarative routing solution that's in place now.