Add constructor fromPtr() for all flutter class with factory ctor. (#1667)

Co-authored-by: wangrui <wangrui@oecore.com>

flutter/sherpa_onnx/lib/src/offline_recognizer.dart

@@ -227,6 +227,8 @@ class OfflineRecognizerResult {
 }
 
 class OfflineRecognizer {
+  OfflineRecognizer.fromPtr({required this.ptr, required this.config});
+
   OfflineRecognizer._({required this.ptr, required this.config});
 
   void free() {