'stack cancel' still needs to be implemented

There are some comments/notes and func stubs in the code, but nothing concrete implemented as of yet.
The `cancel` subcommand should look in the configured Azure DevOps instance for any pending releases created for the user, and cancel & abandon one/some/all of them.
Maybe provide a flag/option to 1) cancel the oldest or 2) everything older than 7d or 24h or some other provided time duration or 3) everything, period, no matter how new/old?