test: add unit test

[marcalexiei]

• Preparing for Update bot to handle new tool structure from changesets and @manypkg/get-packages@3 #97

This PR:

• restores tests in index.test.js using mocks
• adds get-changed-packages.test.ts
• adds ci.yml

[vercel]

@marcalexiei is attempting to deploy a commit to the Changesets Team on Vercel.

A member of the Team first needs to authorize it.

[marcalexiei]

Could you please enable the GitHub Actions in this repo so we can test if the ci workflow works?

[Andarist]

I'm not actually sure how to do it. They are enabled already - but the repo require approval for first-time contributors... but I don't see a way to approve this run.

Anyway, don't worry about it. I'm working on a small revamp here and I'll land this soon! Thank you for your work ❤️