Allow pushing to another branch / manually pushing

Due to the way GitHub Actions work, it's currently impossible to use this plugin on a branch that has required status checks. Hopefully this will change, but in the meantime it would be useful if we could control the pushing more granularly.

Specifically what I would like to do to work around the GHA limitations is to:

1. cut the new release
2. push the resulting commit to a new branch
3. open a PR
4. use mergify to auto-merge it back in.

The problem is that I can't easily do step 2, because this package has no configuration (at least none that I could find) to control the pushing of the commit. It would be useful if there was a setting to either control which branch gets pushed to, or, even better, just a setting to say "don't push". That way the commit could be made automatically, but then we have to deal with it manually afterwards, which allows for much finer control.

Start by tracing the plugin's existing commit and push behavior and its configuration entry points. Determine how to support selecting a target branch or suppressing the push while still creating the release commit. Done means either workflow can create the commit and then allow the user to open and merge a PR manually.