auto-publish unstable packages to NPM

[SergeAstapov]

As there's no convenient way to point NPM directly at a monorepo, and even if you could the package does have a required build step.

The build step for v2 addons was a deliberate design choice, because in the normal case apps can have much faster builds if all their addons are already prebuilt before publishing. But it does make the case of using unreleased code more manual.

This PR adds workflow to automatically publish alpha releases to NPM for every commit to the master branch.

This is inspired by setup in https://github.com/ember-animation/ember-animated/blob/master/.github/workflows/publish-unstable.yml

@lukemelia would you be open doing this? If yes - could you please add npm token as NODE_AUTH_TOKEN secret to the repository config?

[lukemelia]

If yes - could you please add npm token as NODE_AUTH_TOKEN secret to the repository config?

Added 👍

[SergeAstapov]

8.1.1-unstable.5290a3b was successfully published!