Atomic pre_release()

[krlmlr]

There are a few ways where pre_release() can fail:

• [ ] GITHUB_PAT not set (should we use gitcreds)?
• [ ] The remote is not a GitHub remote (we should leave out the PR creation but otherwise continue)
• [ ] Not on main branch
• [ ] Main branch not up to date
• [ ] cran-x.y.z branch exists locally
• [ ] cran-x.y.z branch exists on the remote (a failed pre_release() attempt perhaps?)
• [ ] A pull request for the branch already exists
• [ ] Tag vx.y.z exists locally
• [ ] Tag vx.y.z exists on the remote
• [ ] Version >= x.y.z already on CRAN
• [ ] ...

Ideally we would do a preflight check that aborts early without taking any action. Alternatively, we could support an undo function that cleans up everything to the state it was just before the attempted release.