`gh stack list` - show me my stacks

The closest thing to this is `gh stack checkout` which opens an interactive prompt to checkout a different stack. But all I want to do is list my stacks. Thanks!

Start by reading the implementation of `gh stack checkout`, the closest existing stack-related entry point mentioned in the issue. Add a `gh stack list` command that lists the user's stacks without opening an interactive prompt, and verify the command's output against the available stacks.

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