clipboard support in read mode

[jaysonss]

Steps to Reproduce

In Preview mode, TextSelection is not support.

Logs

[✓] Flutter (Channel stable, 2.0.1, on macOS 11.5.2 20G95 darwin-x64, locale
zh-Hans-CN)
• Flutter version 2.0.1 at /Users/huangyumeng/flutter/sdk/flutter
• Framework revision c5a4b4029c (6 months ago), 2021-03-04 09:47:48 -0800
• Engine revision 40441def69
• Dart version 2.12.0
• Pub download mirror https://pub.flutter-io.cn
• Flutter download mirror https://storage.flutter-io.cn

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
• Android SDK at /Users/huangyumeng/Library/Android/sdk
• Platform android-30, build-tools 29.0.2
• Java binary at: /Applications/Android
Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build
1.8.0_242-release-1644-b3-6915495)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 12.5.1, Build version 12E507
• CocoaPods version 1.10.1

[✓] Chrome - develop for the web
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.1)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/6351-dart
• Java version OpenJDK Runtime Environment (build
1.8.0_242-release-1644-b3-6915495)

[✓] IntelliJ IDEA Community Edition (version 2020.1.2)
• IntelliJ at /Applications/IntelliJ IDEA CE.app
• Flutter plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/6351-dart

[✓] VS Code (version 1.57.1)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.22.0

[✓] Connected device (4 available)
• Redmi Note 8 Pro (mobile) • p75xzxv8dqyxozqg •
android-arm64 • Android 10 (API 29)
• iPhone (mobile) • 07764bc3e54f20576249b16f73d1a6311787f474 • ios
• iOS 14.7.1
• macOS (desktop) • macos •
darwin-x64 • macOS 11.5.2 20G95 darwin-x64
• Chrome (web) • chrome •
web-javascript • Google Chrome 92.0.4515.159

• No issues found!