Manifest: add an alternative dune file

What & Why

Adds a simple dune file for the manifest, so that ocaml-lsp users can benefit from it when editing the manifest. It actually produces a valid main.exe.

How

• Adds a dune file
• Update the README

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR