Add Flutter text to speech demo (#1087)

flutter-examples/tts/README.md

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+# tts
+
+This example demonstrates how to use text to speech (TTS) in Flutter with sherpa-onnx.
+
+It works on the following platforms:
+
+  - Android
+  - iOS
+  - Linux
+  - macOS (both arm64 and x86_64 are supported)
+  - Windows
+
+## How to build
+
+Before you run `flutter build`, you have to select a TTS model and change
+the code to use your selected model.
+
+### 1. Select a TTS model
+
+We have a list of TTS models at
+
+<https://github.com/k2-fsa/sherpa-onnx/releases/tag/tts-models>
+
+You can select any of them. If you feel that there are so many that you don't know
+which one is the best, please visit <http://huggingface.co/spaces/k2-fsa/text-to-speech>
+and try each one by yourself and select the one you consider the best.
+
+Suppose you select
+
+  <https://github.com/k2-fsa/sherpa-onnx/releases/download/tts-models/vits-piper-en_US-libritts_r-medium.tar.bz2>
+
+Then please do the following:
+
+  - 1. Download and unzip the model
+
+```bash
+cd flutter-examples/tts/assets
+wget 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
+cd ..
+
+./generate-asset-list.py
+```
+
+  Note that you have to run [./generate-asset-list.py](./generate-asset-list.py) so that Flutter knows where
+  to find the model.
+
+  - 2. Change the code to use the downloaded model.
+
+    We have given several examples for different models in [./lib/model.dart](./lib/model.dart).
+    For our selected model, we need to change [./lib/model.dart](./lib/model.dart) so that it looks like below:
+
+```
+// Example 6
+// https://github.com/k2-fsa/sherpa-onnx/releases/tag/tts-models
+// https://github.com/k2-fsa/sherpa-onnx/releases/download/tts-models/vits-piper-en_US-libritts_r-medium.tar.bz2
+modelDir = 'vits-piper-en_US-libritts_r-medium';
+modelName = 'en_US-libritts_r-medium.onnx';
+dataDir = 'vits-piper-en_US-libritts_r-medium/espeak-ng-data';
+```
+
+  - 3. That's it.
+
+### Build the APP
+
+  - 1. For Linux
+
+```bash
+flutter build linux
+
+# See below if you get any errors
+```
+
+  - 2. For macOS
+
+To build a universal2 APP, use
+
+```bash
+flutter build macos
+```
+
+To build for `x86_64`, use
+
+```bash
+export FLUTTER_XCODE_ARCHS=x86_64
+flutter build macos
+```
+
+To build for `arm64`, use
+
+```bash
+export FLUTTER_XCODE_ARCHS=arm64
+flutter build macos
+```
+
+  - 3. For Windows
+
+```bash
+flutter build windows
+```
+
+  - 4. For Android
+
+```bash
+flutter build apk --split-per-abi
+```
+
+  - 5. For iOS
+
+```
+flutter build ios
+```
+
+## Fix for Linux
+
+If you get the following errors on Linux,
+
+```
+Building Linux application...
+CMake Error at /usr/local/share/cmake-3.29/Modules/FindPkgConfig.cmake:634 (message):
+  The following required packages were not found:
+
+   - gstreamer-1.0
+
+Call Stack (most recent call first):
+  /usr/local/share/cmake-3.29/Modules/FindPkgConfig.cmake:862 (_pkg_check_modules_internal)
+  flutter/ephemeral/.plugin_symlinks/audioplayers_linux/linux/CMakeLists.txt:24 (pkg_check_modules)
+```
+
+please run:
+
+```bash
+sudo apt-get install -y libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev libunwind-dev
+```
+
+See also <https://github.com/bluefireteam/audioplayers/tree/main/packages/audioplayers_linux#setup-for-linux>
+for the above error.