[wizard_router] Allow enums for route names

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.

```dart
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()),
},
);
```

Start by locating the wizard_router package's Wizard and WizardRoute definitions and the existing string-based route API. Check how route names are stored, passed to the widget, and used for navigation. Done means enum-keyed routes are supported without removing the existing string API, with coverage for both forms.

Written by the indexing model from the issue text.