Upgrade mode

[fhammerschmidt]

Would be cool if we had some mechanism like
npm create rescript-app --upgrade
that guides users through the process of upgrading rescript. We already read the package.json here, why not do the same with the rescript.json and determine if even a fully automated upgrade is possible? At least it could be semi-automated and return the user a link to the correct migration guide(s).

This probably makes more sense to implement when the migration guides are done: rescript-lang/rescript-lang.org#945