feat(newsboat): add key for syncing, add indicator to bar

lib/i3status-rust.nix

@@ -85,6 +85,11 @@ in {
     };
   };
   block = [
+    {
+      block = "custom";
+      interval = 5;
+      command = "newsboat-unread-count";
+    }
     {
       block = "weather";
       autolocate = true;