Add publish dry run job

This adds a publish_npm_packages_dry_run job that runs only in merge request pipelines.

It displays which packages would be bumped by the MR, and a diff of the changes to package.json/changelog files that would be made.

Example output

• Bumping multiple packages
• Bumping one package
• Bumping no packages

Note

This isn't a big MR, but you might find it easier to review commit-by-commit.