feat: simple tailscale module to persist its directory

modules/nixos/vpn/default.nix

@@ -0,0 +1,4 @@
+{ ... }:
+{
+  imports = [ ./tailscale.nix ];
+}

modules/nixos/vpn/tailscale.nix

@@ -0,0 +1,23 @@
+{
+  config,
+  lib,
+  ...
+}:
+let
+  inherit (lib) mkOption mkIf;
+  inherit (lib.types) bool;
+  cfg = config.ataraxia.vpn.tailscale;
+in
+{
+  options.ataraxia.vpn.tailscale = {
+    enable = mkOption {
+      type = bool;
+      default = config.services.tailscale.enable;
+      description = "Enable tailsacle";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    persist.state.directories = [ "/var/lib/tailscale" ];
+  };
+}