Autoformat generated files before diffing/promotion

This could be done by adding an optional styler field to the stanza but I'm not sure that's the best solution here.

An alternative is to allow overwriting the action, just as with the test stanza but I'm not sure that's great either, that's what I attempted at first but I realized that while it wasn't allowed by dune I had no idea how it was invoked and what were the right set of dependencies.

It would be nice if it used ocamlformat by default if the project is already using it but I think it makes sense to allow to pass stylers even when that's not the case. Relying on an external formatter so that you don't have to bother with that in your code generating code is what's most important about this feature.