feat: automated pub publish workflow

[felangel]

As a Dart/Flutter package developer, I want to have a reusable workflow that will automated the publish to pub.dev.

[BeatriceMitchell]

This still needs context to determine what the trigger for automated workflow should be.

[BeatriceMitchell]

This will be manually triggered when publishing is desired.

[renancaraujo]

After some exploration and conversation, we decided that this is a wontfix.

Reasons: 1 - There is a matter of what value we are giving. There are some actions that take care of publishing and generating a changelog automatically is a too error-prone process. 2 - Not everything can be automated: a tag has to be defined manually.