Add sharezone_lints to notifications package

[nilsreichardt]

This PR fixes the following warnings:

Analyzing notifications...                                              

   info • Use the generic function type syntax in 'typedef's • lib/src/action_registration.dart:24:24 • prefer_generic_function_type_aliases
   info • The imported package 'meta' isn't a dependency of the importing package • lib/src/instrumentation.dart:9:8 •
          depend_on_referenced_packages
   info • The local variable '_instrumentation' starts with an underscore • lib/src/notification_parser.dart:21:50 •
          no_leading_underscores_for_local_identifiers
   info • The imported package 'collection' isn't a dependency of the importing package • lib/src/push_notification.dart:9:8 •
          depend_on_referenced_packages
   info • Unnecessary 'this.' qualifier • lib/src/push_notification.dart:51:8 • unnecessary_this
   info • The local variable '_other' starts with an underscore • lib/src/push_notification_action_handler.dart:216:43 •
          no_leading_underscores_for_local_identifiers
   info • The local variable '_duplicates' starts with an underscore • lib/src/push_notification_action_handler.dart:223:29 •
          no_leading_underscores_for_local_identifiers
   info • Don't invoke 'print' in production code • test/docs_example_test.dart:24:9 • avoid_print
   info • Use 'const' with the constructor to improve performance • test/docs_example_test.dart:36:7 • prefer_const_constructors

9 issues found. (ran in 1.4s)

Part of #37

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[Jonas-Sander]

See above

[Jonas-Sander]

Well the thing is this is the code example for the documentation of the PushNotificationActionHandler.
So I guess the example in the doc also needs to be updated, altough I think it would be more confusing to use log instead of print in the example.
So maybe just add ignore_for_file: avoid_print to the top of this file?

[nilsreichardt]

Yes, makes sense. I added it in 1f6b5c7