🚀 Feature: Add a way to apply known file changes after migration

[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, if you want to run npx create-typescript-app --mode migrate on an existing repository that doesn't use exactly what the template sets up, there'll likely be some changes to tooling. For example, a GitHub Action that uses ncc instead of tsx to build might see a change like:

// package.json
- "build": "tsx"
+ "build": "tsx"
...
+ "ncc": "^1.2.3",
- "tsx": "^1.2.3",

This is annoying and inconvenient. I'd like to allow repositories to store their own description of changes to be made after migration. That way they can run migration mode, then those changes - and end up with no unintended changes.

I suspect there may be two kinds of changes to be made:

• Terminal commands: running installs, uninstalls, and the like
• Text (diff): directly modifying built files

Additional Info

This is essentially a re-file of #498. Spinning out into a new issue as the comment history on that one includes a good bit of discussion to get us to this point.

This is essentially turning the end-to-end test snapshots into a real feature. Better documenting those snapshots is tracked in #1141.

There's also a similarity to how https://github.com/ds300/patch-package allows applying changes after installation...