Add TTS API and examples for Dart (#1010)

dart-api-examples/tts/run-piper.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+set -ex
+
+dart pub get
+
+
+# Please visit
+# https://github.com/k2-fsa/sherpa-onnx/releases/tag/tts-models
+# to download more models
+
+if [[ ! -f ./vits-piper-en_US-libritts_r-medium/tokens.txt ]]; then
+  curl -SL -O https://github.com/k2-fsa/sherpa-onnx/releases/download/tts-models/vits-piper-en_US-libritts_r-medium.tar.bz2
+  tar xf vits-piper-en_US-libritts_r-medium.tar.bz2
+  rm vits-piper-en_US-libritts_r-medium.tar.bz2
+fi
+
+dart run \
+  ./bin/piper.dart \
+  --model ./vits-piper-en_US-libritts_r-medium/en_US-libritts_r-medium.onnx \
+  --tokens ./vits-piper-en_US-libritts_r-medium/tokens.txt \
+  --data-dir ./vits-piper-en_US-libritts_r-medium/espeak-ng-data \
+  --sid 351 \
+  --speed 1.0 \
+  --text 'How are you doing? This is a speech to text example, using next generation kaldi with piper.' \
+  --output-wav piper-351.wav
+
+ls -lh *.wav