feat(dune): support + prefixes for PPX CLI flags

[anmonteiro]

this is useful for cases like melange.ppx which emit alerts and allow to configure them via -alert ....

[rgrinberg]

I think this needs to be versioned. You can version this under dune's own language or melange's. Depending on how stable you'd like to keep this feature.

[anmonteiro]

Why do you think this needs to be versioned?

I believe the previous behavior would just plainly return an error, so we shouldn't have anyone depending on these features, right?

[rgrinberg]

Suppose that somebody would try to use this feature in a publicly released project. That person would need to remember to update the constraints in their project file to require at least dune 3.18. Without the version check, it is very likely that the person would forget to add this constraint, or they might even do it incorrectly, if they misremember which version of dune introduced this feature. Downstream user of this project might then encounter build failures should they build with an older version of dune. If we add a version check, we make this scenario more or less impossible.

It's true that the above argument is applicable to basically any user visible change in dune, but I think it works best for features. As for bug fixes, it's good that users can benefit from those without opting in. I think that this PR is certainly more of a feature than a bug fix.

[anmonteiro]

absolutely, thanks for the explainer!

[anmonteiro]

@rgrinberg ready for another look.

[rgrinberg]

I think you could improve the error message here with a hint for prefix = "+" when syntax_version < (3, 18)