bump to 4.2.0

pubspec.yaml

 name: fluwx
 description: The capability of implementing WeChat SDKs in Flutter. With Fluwx, developers can use WeChatSDK easily, such as sharing, payment, lanuch mini program and etc.
-version: 4.1.1+1
+version: 4.2.0
 homepage: https://github.com/OpenFlutter/fluwx
 
 environment: