Ease pull requests with a GitHub CI workflow

[nfreear]

Hi,

To ensure that pull requests, like those for #56, won't break things, I suggest that we add a GitHub workflow to the repository.

This is a generic Node.js starter:

• https://github.com/actions/starter-workflows/blob/main/ci/node.js.yml;
• https://nodejs.org/en/about/releases/;

I'll do a pull request!

Thanks,

Nick

---

• — Workflow .github/workflows/nodejs.yml;
• — Badge in the README;
• — (TODO: later, add linting?)