Option to generate workflow files for automated publishing

[jonasfj]

Context: https://dart.dev/tools/pub/automated-publishing

I did this with some hacks in dart-neats:
https://github.com/google/dart-neats/blob/master/.github/workflows/publish.yml

It assumes packages are in a folder with the package name, and relies on a tag pattern like <package>-v{{version}}.

Perhaps it would better if there was an option for mono_repo to generate a workflow file for each package -- it's not as elegant, but a lot less fragile -- or maybe there is some other option we can consider?

[kevmoo]

CC @devoncarew – he added some tooling to https://github.com/dart-lang/ecosystem/blob/main/pkgs/blast_repo/lib/src/tweaks/auto_publish_tweak.dart for this!