feat: Add exists method for checking save game file existence in CLI and Flutter persistence

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

apps/wolf_3d_cli/lib/cli_save_game_persistence.dart

@@ -34,6 +34,19 @@ class CliSaveGamePersistence implements SaveGamePersistence {
     }
   }
 
+  @override
+  Future<bool> exists({
+    required int slot,
+    required GameVersion version,
+  }) async {
+    try {
+      final File file = File(_slotPath(slot, version));
+      return file.existsSync() && file.lengthSync() > 0;
+    } catch (_) {
+      return false;
+    }
+  }
+
   @override
   Future<void> save({
     required int slot,