Dart API examples

Please see

• Decoding a file: https://github.com/k2-fsa/sherpa-onnx/blob/master/dart-api-examples/non-streaming-asr/run-sense-voice.sh

and

• Decoding a file with VAD: https://github.com/k2-fsa/sherpa-onnx/blob/master/dart-api-examples/vad-with-non-streaming-asr/run-moonshine.sh