WECHATY_LOG environment variable specification (valid values)

[huan]

According to our Wechaty Specification https://wechaty.js.org/docs/specs/wechaty:

The WECHATY_LOG should support the values of silly, verbose, info, warn, silent

All Polyglot Wechaty should at least support the above names as an alias.

Link to https://github.com/wechaty/wechaty/issues/2167