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

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

flutter/sherpa_onnx/lib/src/punctuation.dart

@@ -36,6 +36,8 @@ class OfflinePunctuationConfig {
 }
 
 class OfflinePunctuation {
+  OfflinePunctuation.fromPtr({required this.ptr, required this.config});
+
   OfflinePunctuation._({required this.ptr, required this.config});
 
   // The user has to invoke OfflinePunctuation.free() to avoid memory leak.