Github action: add publish npm workflow

[jackw]

Manually publishing to npm and github is a bit of a painful operation with needing to create auth tokens, editing npmrc between publishings, etc. This is a WIP for getting a github action together for publishing this package automatically when a merge to master occurs that contains a change to the version property in package.json.

WIP as I've never worked with github actions before, not sure if this should live in our github-actions repo, and also requires secrets creating for NPM_TOKEN and GITHUB_TOKEN for the workflow to function.

[mckn]

I think this looks great! Is this something we can share across multiple repos? if not then I think we should keep it in this repo.

[jackw]

Yep I'd like to put this somewhere we can share it but I'm not sure how that works. @torkelo do you know if it's possible to share workflows between repos or would I need to re-write this as an action?

[zoltanbedi]

nice! lgtm 🐈

[jackw]

@dprokop I apologise for the ping but could you assist me getting this merged so we can automate publishing this package to npmjs and github repositories? I don't have the permissions to add the secrets required for this publish workflow to run.