Manifest: Create a top level variable for dependencies

The following discussion from !9478 (merged) should be addressed:

• @raphael-proust started a discussion:

  Can you open a follow-up issue to change the way we add our dev dependencies?

  We should probably have a top-level variable that's like

  let project_dev_dependencies = [
    bisect_ppx;
    ocamlformat;
    ometrics;
  ]

  And then add all those dependencies as dev to all packages. Or something like that.