Automate npm releases

[bpierre]

I think it could be nice to consider the master branch stable, and automate the build + version bump + npm publication using GitHub Actions.

We could:

• Have a specific line in the PR and merge commit (e.g. change: major|minor|patch) to determinate the new version.
• Have one of the status checks verifying that the change: information exists (and prevent merging without it).
• For every merge on master, a workflow would:
  • Build everything.
  • Bump the version.
  • Push the newly created tag.
  • Publish on npm.

The changelog will still be edited manually.

Some things we might want to explore:

Using merge-release

Some potential issues:

• It only checks at the commit level, so we can’t run a PR status check to verify that the information has been set.
• It is not as explicit as having a change: major|minor|patch, which might cause issues.
• I couldn’t make it work with aragonUI and our npm account. For some reason, it never accepts to authenticate, even without OTP.

Using GitHub Packages + npm-package-sync

This would allow to publish on GitHub Packages, and just sync it with npm. It would make things easier if GitHub Packages provide a transparent bridge to npm at some point.

/cc @sohkai @promaty