解析json字符串时，取消unicode自动解码

```
String json = "{\"message\": \"\\u4f60\\u597d\"}";
System.out.println(JSON.parse(json));
```
输出结果：{"message":"你好"}
期待的结果：{"message": "\u4f60\u597d"}

根据文档，[https://alibaba.github.io/fastjson2/features_cn.html](https://alibaba.github.io/fastjson2/features_cn.html)
JSONReader.Feature根本没有对应取消解码unicode的实现

我之前有提过，但您一直没有回复，可能是没有看到，所以我关闭了之前的[问题](https://github.com/alibaba/fastjson2/issues/2119)，提出了这个问题。

Start with the JSON.parse example and the JSONReader.Feature documentation linked in the issue. Investigate how escaped Unicode is handled during parsing and whether an existing feature controls that behavior. Done means a documented option can preserve the literal \u4f60\u597d sequence instead of decoding it to Chinese characters, with tests covering the example.

Written by the indexing model from the issue text.