add CI script to publish to npm

[k-yle]

Based on the recent discussions, it sounds like everyone supports the idea of publishing releases.

So, now that #2917 is merged, this PR adds a button to the GitHub UI, which a maintainer can click to publish a new version to npm:

There's no authentication tokens required, and this runs completely in the CI, maintainers don't need to run any commands locally.

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Before this can be merged, there are a few extra steps that a maintainer needs to take (click to expand)

1. Go to /settings/environments/new, and create new environment called npm. Setup whatever restrictions you want, like only allowing deployments from the gh-pages branch.
2. Open the existing npm package on the npm website, click on Settings, and configure the following options:

Once these two steps are done, publishing will work from the GitHub CI without needing to setup any other authentication or permissions. (thanks to the 'trusted publishing' feature)