Publish pre-releases as a separate tag on npm
npm will install the newest version a package has published to the `latest` tag,
including pre-releases, which is not ideal since those may not be ready for
production use yet.
This uses an alternate tag (`next` is a common convention, but it can be
anything) for pre-releases so the default installs only get stable versions.
Fixes https://github.com/vector-im/riot-web/issues/12029
