Decide on a style guide for the openfoodfacts-dart package

[stephanegigandet]

The Flutter team from Google reviewed the Smoothie (OFF new app written in Flutter) code base, and they had some recommendations to improve it. One of them is to adopt a style guide such as https://dart.dev/guides/language/effective-dart/style or https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo (or to modify an existing one to suit our needs)

You can find the full assessment and recommendations and the corresponding discussion here: openfoodfacts/smooth-app#434

I think it may be useful to do it for the openfoodfacts-dart package as well. What do you think?

[MohamedFBoussaid]

@stephanegigandet Yeah this will be very helpful