Chewing C API should not override rust logger if another logger is already set

[kanru]

Is your feature request related to a problem? Please describe.

Currently we always override the logger to ChewingLogger, and underlying logger env_logger. On Windows we might want to override this to win_dbg_logger from C++ but we can't.

Describe the solution you'd like

Detect if another logger is already set, then don't override it.