Add GitHub Actions CI

[onkoe]

It's hard to know whether changes in this repo break something.

Let's add CI to fix that!

Checklist

Complete all of these points to close the issue.

• Create a .github/workflows/ci.yml file
• Install NodeJS using the template suggested here
• Ensure runs-on is set to ubuntu-24.04-arm, NOT ubuntu-latest
  • ubuntu-latest can cause CI to break randomly due to unwanted major updates; you should never use it in your CI.