Add merlin to the devshell, because I use emacs

flake.nix

@@ -17,7 +17,11 @@
       nativeBuildInputs = with pkgs; [
         ocamlPackages.menhir
       ];
-      
+    };
+    devShells.x86_64-linux.default = pkgs.mkShell {
+      nativeBuildInputs = with pkgs.ocamlPackages; [
+        menhir merlin dune_3
+      ];
     };
   };
 }