Add GoRouterDelegateListener class to track screen views

[bizz84]

Replaces #160 by implementing a workaround for this issue:

• [go_router] ShellRoutes seem to cause NavigatorObserver to not fire (5.0.1)

Workaround is based on this comment:

flutter/flutter#112196 (comment)

Example log while navigating the app:

[log] screenName: signIn, pathParams: {}
[log] screenName: jobs, pathParams: {}
[log] screenName: job, pathParams: {id: 6NDSdYl00ZSDTbfY1zoX}
[log] screenName: jobs, pathParams: {}
[log] screenName: entries, pathParams: {}
[log] screenName: profile, pathParams: {}

[atnpem]

Hello,
Doesn't injecting the Firebase observer directly into the “observers” property track all pages?

GoRouter(
        navigatorKey: _rootNavigatorKey,
        observers: [FirebaseAnalyticsObserver(analytics: FirebaseAnalytics.instance)],
        refreshListenable: GoRouterRefreshStream(...),
        routes: [...])