Nested wizards

```dart
Wizard(
routes: {
'/a': WizardRoute(builder: ...),
'/b': WizardRoute(
children: {
'/1': WizardRoute(builder: ...),
'/2': WizardRoute(builder: ...),
'/3': WizardRoute(builder: ...),
},
),
'/c': WizardRoute(builder: ...),
},
)
```

See also:
- canonical/wizard_router#58

Start by reviewing the existing Wizard and WizardRoute APIs, then read the linked canonical/wizard_router#58 discussion for the intended nested-routing behavior. Done means the example's nested children configuration is supported and covered by the project's relevant tests.

Written by the indexing model from the issue text.