refactor(modules/nixos): move stuff around

This makes the module a bit clearer and easier to extend.

modules/nixos/dev.nix

@@ -0,0 +1,18 @@
+{ lib, config, ... }:
+let
+  cfg = config.jconfig.dev;
+in
+{
+  config = lib.mkIf (config.jconfig.enable && cfg.enable) {
+    # Enable dev documentation
+    documentation.dev = { inherit (cfg) enable; };
+
+    users.extraUsers = lib.mkIf cfg.jupyter.enable { jupyter.group = "jupyter"; };
+
+    services.jupyter = {
+      inherit (cfg.jupyter) enable;
+      group = "jupyter";
+      user = "jupyter";
+    };
+  };
+}