refactor: moved nixos modules to dendrite pattern

modules/nixos/programs/tailscale/tailscale.nix

@@ -0,0 +1,21 @@
+{...}: {
+  flake.nixosModules.tailscale-service = {
+    config,
+    lib,
+    ...
+  }: {
+    options = {
+      host.tailscale = {
+        enable = lib.mkEnableOption "should tailscale be enabled on this computer";
+      };
+    };
+
+    config = lib.mkIf config.services.tailscale.enable (
+      lib.mkMerge [
+        {
+          # any configs we want shared between all machines
+        }
+      ]
+    );
+  };
+}