Minor patch to open up access to the logging callback and freeze the enum for LogLevel

Usage:

Logging.shared.loggingCallback = { message in
    print("WhisperKit logs: ", message)
}

What's Changed

• Freeze loglevel enum by @ZachNagengast in https://github.com/argmaxinc/WhisperKit/pull/255

Full Changelog: https://github.com/argmaxinc/WhisperKit/compare/v0.9.3...v0.9.4