[wizard_router] Allow enums for route names

[pierremrtn]

enums would allow compile-time checked routes. This doesn't have to replace the string API, using a generic parameter on the widget, or a different constructor would allow both.

enum WizardRoutes {
  foo,
  bar,
  baz
}

Wizard(
  routes: {
    WizardRoutes.foo: WizardRoute(builder: (context) => FooPage()),
    WizardRoutes.bar: WizardRoute(builder: (context) => BarPage()),
    WizardRoutes.baz: WizardRoute(builder: (context) => BazPage()),
  },
);