Add support for OCaml dune projects

[hongchangwu]

Although relatively new, Dune has become the de-facto standard build tool for OCaml. The vast majority of packages on Opam uses Dune now.

Reference for project root file: https://dune.readthedocs.io/en/stable/overview.html?highlight=dune-project#project-layout

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Thanks!

[bbatsov]

Thanks!