Publish packages automatically after merge

[swift-kim]

Currently the publication is manually done by human. We may add a workflow to the CI that does the following things.

• Check for packages that need publishing (by comparing versions of local packages and packages on pub.dev).
• Set up pub.dev credentials (accessToken and refreshToken).
• Run pub publish --dry-run for each package on pull request.
• Run pub publish for each package on push.

Basic idea: https://medium.com/evenbit/publishing-dart-packages-with-github-actions-5240068a2f7d

[swift-kim]

We could make this a tool command similarly to flutter_plugin_tools' PublishPluginCommand.

We can't use their code as it is, and need to make some modifications. Especially we don't tag our releases, so related checks should be skipped.

[HakkyuKim]

I also looked into this a few days ago, I'll work on this.