Publish to GitHub packages in addition to npmjs

[harmony7]

This PR updates the NPM package publish workflow to publish to GitHub packages in addition to npmjs.

There are a few changes here:

• The proposed change now uses the workflow described in https://docs.github.com/en/actions/use-cases-and-examples/publishing-packages/publishing-nodejs-packages to use the setup-node action to set up the node environment. As a result of this action, the following is added to .npmrc in the runner (including the variable placeholder):

  //registry.npmjs.org/:_authToken=${NODE_AUTH_TOKEN}
  registry=https://registry.npmjs.org/
  always-auth=true
  

• After this we add the following line to .npmrc by invoking npm config set:

  //npm.pkg.github.com/:_authToken=${NODE_AUTH_TOKEN}
  

• Now, when publishing to npmjs, we set the environment value NODE_AUTH_TOKEN to the NPM publish token, which is resolved as npm publish reads .npmrc.

• After that, there is a new step that publishes to GitHub packages, which sets the registry for the @fastly scope to npm.pkg.github.com. For this step we set the environment value NODE_AUTH_TOKEN to the GitHub token, which is resolved as npm publish reads .npmrc.

[harmony7]

Thanks for the quick review, @Integralist !

I'm also going to ask for @kpfleming to give it a once over, and then we'll merge it.