Add log level of warn

[weizhengde]

In many other logging frameworks, log levels always include debug, info, warn, error. However, in go-zero framework, log levels only include debug, info, error, severe. In many business scenarios, warn level logs are more semantic.

[Sebastian-Getts]

Maybe Alert Level can be used

[shenpuyu0]

我确实需要warn级别的日志
增加warn后，可以更好的过滤出需要注意而不影响程序运行的日志
部分第三方库支持传入logger，但定义的接口大多都包含debug、info、warn、error四个级别， 增加warn后就免去了自定义logger的麻烦
希望能考虑一下

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I do need warn level logging
After adding warn, you can better filter out logs that require attention without affecting the running of the program.
Some third-party libraries support incoming loggers, but most of the defined interfaces include four levels: debug, info, warn, and error. Adding warn eliminates the trouble of customizing the logger.
Hope you can consider it