ocamlPackages.merlin: add meta.mainProgram (NixOS#397595)

Co-authored-by: Arne Keller <[email protected]>

Author: Patrick-Poitras

pkgs/development/tools/ocaml/merlin/4.x.nix

@@ -95,6 +95,7 @@ buildDunePackage {
     description = "Editor-independent tool to ease the development of programs in OCaml";
     homepage = "https://github.com/ocaml/merlin";
     license = licenses.mit;
+    mainProgram = "ocamlmerlin";
     maintainers = [
       maintainers.vbgl
       maintainers.sternenseemann