Allow list expansion in preprocessor arguments

[Firobe]

This PR allows templates to expand into multiple values in preprocessor flags, like:

 (preprocess (pps my_ppx -- arg1 arg2 %{read-lines:my-ppx-flags} arg8 arg9))

This makes it consistent with similar usage of read-lines for run to pass correctly separated arguments.

[rgrinberg]

Thanks for the PR. We need a few more changes to merge it though:

• A test demonstrating the issue
• Guarding the new behavior behind a version check. The old behavior should be preserved if the project version is 3.2 or less.

If you have any questions on how to do the above, please don't hesitate to ask.

[Firobe]

Hey @rgrinberg, I have a test but I'm not against some help to retrieve the current dune lang version, in particular how to escape the Decoder monad if using that interface? I imagine we have to pass it from elsewhere, but I'm not sure where :)

[rgrinberg]

You can obtain the version from Scope.project scope |> Dune_project.dune_version. I think the caller has the scope so you should be able to pass in the version to the function you're modifying.

[Firobe]

Thank you! I think the PR is ready to be reviewed again.

For the test, I tested the new behavior in a separate test for the (yet non-existent) version 3.3, after checking that the behavior actually works if there is no version guard. I'm not sure if you have a process for writing tests for the version currently in development.

[rgrinberg]

Merged. Thanks.