Flutter add official support to firebase crashlytics

https://pixabay.com/illustrations/software-testing-service-762486/

One year and 447 👍 later, the official Firebase Crashlytics plugin has been released. Right now it does not mean much the plugin still has the same limitations of the flutter_crashlytics, an unofficial plugin. The main one is that the crashes are reported as non-fatal. The native firebase crashlytics library does not support specifying if the crash is fatal or not, so this need to be fixed by the crashlytics team first. With the official plugin, we hope that crashlytics team prioritize this issue and help flutter devs to have a first class integration with firebase.