Make `dune init proj` create the `dune-project` file

[shonfeder]

Followup to #4239

It should also populate the dune-project file with useful stanzas following our recommended best practices, such as:

• Current lang version
• Enable opam file generation
• Minimal package definition

This has also come up on the discourse: https://discuss.ocaml.org/t/defining-standard-ocaml-development-lifecycle-processes/7486/22

That sounds good. In today's meeting, we also discussed the fact that just dune init should create a simple dune-project file (cf #4459).

[shonfeder]

@jeremiedimino Would you be ok with an implementation that introduced a function Dune_project.to_dune_lang : Dune_project.t -> Dune_lang.t? This seems like the most principled way, as opposed to coding up an adhoc set of Dune_lang.t stanzas, that might get out of sync with the Dune_project. (We do the later for the library, executable, and test stanzas currently, but those are much shorter, and this is also not preferable.)

That seems fine to me. We generally call such functions encode BTW.