Basic GitHub actions support for Developer Preview (ala setup-ocaml)

[mbarbin]

In addition to testing local workflows, it would be useful to me to be able to test some CI workflows for OCaml projects on GitHub.

Would you consider creating a basic template that can install the dune developer preview from the nightly builds, and be able to compile in an actions file a project that has an existing dune.lock config?

I'm thinking that I could probably consider contributing and provide feedback if there was some central place somewhere that could act as a template for the community to get started.

Thanks for your consideration!