WECHATY_LOG environment variable specification (valid values)

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