Updated to Flutter 3 and general cleanup

[novas1r1]

Hello :)!
Since I'm using the package in my current app and it is not yet supporting Flutter 3 I added this PR. I also adjusted the package to match the current valid flutter package configuration. I also added flutter_lints to it.

Feel free to merge or use my branch it in your project:

flutter_time_picker_spinner:
    url: https://github.com/novas1r1/flutter_time_picker_spinner
    ref: master

[ScottishRoss]

Thanks for this!

Note for anyone else, you need to add this line:

flutter_time_picker_spinner:
    git:
      url: https://github.com/novas1r1/flutter_time_picker_spinner
      ref: master