feat: expose YaruNavigationPage.navigatorKey and onGenerateRoute

[jpnurmi]

Allows pushing (named) routes onto YaruNavigationPage's internal Navigator. For example:

YaruNavigationPage(
  navigatorKey: _navigatorKey,
  ...
  onGenerateRoute: (settings) => switch (settings.name) {
    '/detail' => MaterialPageRoute(
        settings: settings,
        builder: (_) => DetailPage(settings.arguments as Detail),
      ),
    '/search' => MaterialPageRoute(
        settings: settings,
        builder: (_) => SearchPage(query: settings.arguments as String),
      ),
    _ => null,
  },
)

From within the navigation page:

Navigator.pushNamed(context, '/detail', arguments: detail);

From anywhere else such as a global search bar:

_navigatorKey.currentState!.pushNamed('/search', arguments: query);

[d-loose]

LGTM 👍