🚀 Feature: Add an option to skip file system changes

[JoshuaKGoldberg]

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

Right now, you can use this template in a way that skips API calls (e.g. --offline/--skip-github-api)... but if you just want to update the API calls for an existing repo, the steps are a little manual:

1. Run --mode migrate locally
2. Reset all file system changes in Git

It'd be nice to have option(s) to skip making any changes to the file system, so you don't need to run step 2.

Additional Info

I wonder if this should be split into >=2? Core file system changes and also allcontributors generation...?

[JoshuaKGoldberg]

Repos I already want to run this on 😄:

• https://github.com/JoshuaKGoldberg/commit-to-co-authors
• https://github.com/JoshuaKGoldberg/co-author-to-username