Add CD with GitHub Actions

[willrowe]

This would allow new releases to be automatically created.

• Require 2FA or automation tokens on the npm package.

[willrowe]

This may have to be pushed to a later release and done manually until then. I looked into how to set this up, but it requires an npm user to generate an access token and enter it into the repository secrets. This would be fine if the access token could be restricted to just the twig.js package, but it actually would be able to publish an packages that the user who generated the token has access to, which I am not comfortable with.

Ability to restrict access tokens is being considered by GitHub/npm, so if it is ever added then this could move forward.

Relevant issues:
npm/roadmap#10
github/roadmap#557