Support WebAssembly for text-to-speech (#577)

wasm/assets/.gitignore

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+*.onnx
+*.txt
+espeak-ng-data
+

wasm/assets/README.md

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+# Introduction
+
+Please refer to
+https://github.com/k2-fsa/sherpa-onnx/releases/tag/tts-models
+to download a model.
+
+The following is an example:
+```
+cd sherpa-onnx/wasm/tts/assets
+
+wget -q 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
+mv vits-piper-en_US-libritts_r-medium/en_US-libritts_r-medium.onnx ./model.onnx
+mv vits-piper-en_US-libritts_r-medium/tokens.txt ./
+mv vits-piper-en_US-libritts_r-medium/espeak-ng-data ./
+rm -rf vits-piper-en_US-libritts_r-medium
+```
+
+You should have the following files in `assets` before you can run
+`build-wasm-simd.sh`
+
+```
+assets fangjun$ tree -L 1
+.
+├── README.md
+├── espeak-ng-data
+├── mode.onnx
+└── tokens.txt
+
+1 directory, 3 files
+```