Given a Packagist package identifier and 2 version identifiers, give to the user the list of changes that occurred between these versions.
The goal here is to make it easy to check whether upgrading a package will have an impact or not on your code.
We assume that the packages aren't available locally and thus will require to be pulled using composer. A temporary folder could be created and composer could be used (pointing to this custom directory) to fetch the target packages.

In the long term we might want to plug into composer's outdated command, so that we can check only the packages within the project's composer.json file.