Add a Firebase lint: only_use_instanceFor

[bvoq]

For larger projects using Firebase we most likely don't want to use:

FirebaseAnalytics.instance FirebaseCrashlytics.instance ...

and instead use
FirebaseAnalytics.instanceFor FirebaseCrashlytics.instanceFor ...

It would be neat, if flutterfire provided a lint for this that could be enabled by a project maintainer.