[go_router_builder][go_router] Add support for relative routes

[ThangVuNguyenViet]

Add supports for [go_router_builder] relative routes by adding GoRouter.goRelative which enables TypedRelativeGoRoute to be used as relative route.

Previously it was impossible to construct relative route trees with TypedGoRouter, because the generator will generate duplicate extension class. The only options were either don't use go_router_builder, or (worse) declare each sub-route classes, e.g

@TypedGoRoute<HomeRoute>(
  path: '/',
  routes: [@TypedGoRoute<DetailRouteFromHome>(path: 'details',
    routes: [@TypedGoRoute<SettingsRouteFromDetailFromHome>(path: 'settings')]
  ),
],
)

Now with TypedRelativeGoRoute added, we can declare a common sub-route

/// The route configuration.
final GoRouter _router = GoRouter(
  debugLogDiagnostics: true,
  routes: $appRoutes,
);
const detailRoute = TypedRelativeGoRoute<DetailsRoute>(
  path: 'details/:detailId',
  routes: [
    TypedRelativeGoRoute<SettingsRoute>(path: 'settings/:settingId'),
  ],
);

@TypedGoRoute<HomeRoute>(
  path: '/',
  routes: [detailRoute],
)
class HomeRoute extends GoRouteData {
  @override
  Widget build(BuildContext context, GoRouterState state) {
    return const HomeScreen();
  }
}

@TypedGoRoute<DashboardRoute>(
  path: '/dashboard',
  routes: [detailRoute],
)
class DashboardRoute extends GoRouteData {
  @override
  Widget build(BuildContext context, GoRouterState state) {
    return const DashboardScreen();
  }
}

And simply use DetailsRoute(detailId: 'dId').goRelative(context)

Fixes #108177

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[ThangVuNguyenViet]

Couple of notes here: I can't either add add tests or example for go_router_builder because it depends on the newly added TypedRelativeGoRoute in go_router. Further instructions on this matter is appreciated

[chunhtai]

you need to separate out to two pr, one for go_router another one for go_router_builder. Merge go_router pr first and then you can bump the go_router version in this package so that you can write test and example for it

[ThangVuNguyenViet]

gotcha. Imma temporarily close this PR and open a new go_router only PR and reopen this once the new PR is merged