nix: add a separate shell.nix so I can specify merlin there without adding it as a build requirement

flake.nix

@@ -12,5 +12,6 @@
   in
   {
     packages.default = pkgs.callPackage ./default.nix {};
+    devShells.default = pkgs.callPackage ./shell.nix {};
   });
 }

shell.nix

@@ -0,0 +1,7 @@
+{pkgs ? import <nixpkgs> {}, ...}:
+# I use emacs and merlin while developing
+
+pkgs.mkShellNoCC {
+  inputsFrom = [(import ./default.nix {pkgs=pkgs;})];
+  packages = [pkgs.ocamlPackages.merlin];
+}