Add `--allow-downgrade` option to set-version command

I would like to add an option to allow downgrade version when using `set-version` command.

In my use case this feature is necessary since our CI/CD needs to create dev packages (deb/rpm) for already released versions.

I have already created [PR](https://github.com/killercup/cargo-edit/pull/862) for this feature (Sorry about that, I misunderstood contribution guidelines :confused:)

Review PR #862 first, then inspect the `set-version` command entry point. Confirm that the requested `--allow-downgrade` option supports creating development packages for already released versions and check the existing command behavior before determining what remains.

Written by the indexing model from the issue text.