stack: add basic formatter and label support This still is not great, especially I would like '-T' to be able to control how we display the changeset, but this is useful progress.

stack: show the currently active changeset and unstable ones Still super basic, but give a basic idea of the feature. We use both symbols and explicit text because symbols are cool but text is more explicit

stack: add "t" prefix to index in the output This make the existance and meaning of "t2" reference clearer.