implemented nixos and home-manager nix modules for the flake

nix/module.nix

@@ -0,0 +1,21 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.programs.fern;
+in
+{
+  options.programs.fern = {
+    enable = lib.mkEnableOption "fern shell";
+
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = pkgs.fern;
+      description = "The fern package to use";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    environment.systemPackages = [ cfg.package ];
+    environment.shells = [ cfg.package ];
+  };
+}