Create changelog for package releases

[tidoust]

The NPM packages currently ship without a changelog. It should be relatively easy to create one automatically though, with some diff-ing logic (starting point exists in build-diff.js).

At a minimum, the changelog could link to the corresponding pre-release pull request, which includes the diff from the previous version.

It should be possible to amend the changelog manually, e.g. to override the automatically generated message in situations where we add a new property or change a setting that affects a large number of entries.