Add VAD example for Dart API

[csukuangfj]

We have published the package to pub.dev.

You can find it at
https://pub.dev/packages/sherpa_onnx

The package contains pre-built libs for macOS, Linux, and Windows. So users don't need to build sherpa-onnx from source.

---

The example works on Linux and macOS. However, it throws the following error on Windows:

cd dart-api-examples
  cd vad
  dart run .\bin\vad.dart --silero-vad .\silero_vad.onnx --input-wav .\lei-jun-test.wav --output-wav .\lei-jun-test-no-silence.wav
  
Unhandled exception:
Invalid argument(s): Failed to load dynamic library 'D:\a\_temp\pub-cache\hosted\pub.dev\sherpa_onnx-0.0.3\lib\..\windows\sherpa-onnx-c-api.dll': The specified module could not be found.
 (error code: 126)
#0      _open (dart:ffi-patch/ffi_dynamic_library_patch.dart:11:[4](https://github.com/csukuangfj/sherpa-onnx/actions/runs/9499323914/job/26179977239#step:8:4)3)
#1      new DynamicLibrary.open (dart:ffi-patch/ffi_dynamic_library_patch.dart:22:12)
#2      _dylib.<anonymous closure> (package:sherpa_onnx/sherpa_onnx.dart:43:29)
#3      _dylib (package:sherpa_onnx/sherpa_onnx.dart:48:2)
#4      _dylib (package:sherpa_onnx/sherpa_onnx.dart)
#[5](https://github.com/csukuangfj/sherpa-onnx/actions/runs/9499323914/job/26179977239#step:8:5)      initBindings (package:sherpa_onnx/sherpa_onnx.dart:52:27)
#6      initSherpaOnnx (file:///D:/a/sherpa-onnx/sherpa-onnx/dart-api-examples/vad/bin/vad.dart:29:15)
<asynchronous suspension>
#[7](https://github.com/csukuangfj/sherpa-onnx/actions/runs/9499323914/job/26179977239#step:8:7)      main (file:///D:/a/sherpa-onnx/sherpa-onnx/dart-api-examples/vad/bin/vad.dart:33:3)
<asynchronous suspension>

please see
https://github.com/csukuangfj/sherpa-onnx/actions/runs/9499323914/job/26179977239#step:8:1
for details.

---

Would be great if someone familiar with Dart can look into it.