MVP: reusable workflow

[dominykas]

I initially asked to create this repo so that we can try out the composite actions, but Github finally came out with reusable workflows 🎉 a few days ago, so I figure the MVP here is actually a reusable workflow.

Here's the things I think it should support for the very first draft:

• generate a matrix based on an upgrade-policy
  • policy keywords same as ci-config-travis: all, lts (default), lts-strict
  • detect the minimum version based on engines.node in package.json; fail if undefined/invalid
• actions/checkout
• actions/setup-node
• run npm install
• run npm test
  • pass an environment variable to npm test when testing in latest version so that folks can skip linting for non-latest versions

Usage example:

jobs:
  test:
    uses: pkgjs/action/.github/workflows/test.yml@main
    with:
        upgrade-policy: lts # optional

I should be able to PR today (or before Monday anyways).

---

I'm sure there will be other features we'll want to support (e.g. passing through caching params to setup-node, customizing npm version, etc - basically anything), but I'd like to get the very basics out asap, so that we still have time to iterate on it a little bit before Node.js 16 turns LTS / Node.js 17 comes out and people start updating their workflows manually.

[mcollina]

This would be amazing.

[ljharb]

I would be very interested to see how this works; writing new GitHub actions is very difficult and an example would be helpful :-)

[dominykas]

This is how you use it: https://github.com/pkgjs/action/blob/66bf678e530a693e59a340342165238d8049e67e/.github/workflows/ci.yaml (once #2 gets merged)

Gives you a matrix with lts versions, where oldest is the one from your engines.

[ljharb]

hm, ok - so basically you make one job in the consumer, and it can delegate the entire thing, including using a matrix, to the upstream? i'll give that a shot.