migrate LogicalKeySet to SingleActivator in shorcuts

[yellowgraysu]

Related to flutter/flutter#80756

I want to remove some key in shortcuts.
The following code does not work because the type of shortcuts in VRouter is Map<LogicalKeySet, Intent> but the type of
WidgetsApp.defaultShortcuts is Map<ShortcutActivator, Intent>

VRouter(
    shortcuts: Map.from(WidgetsApp.defaultShortcuts)..remove(
        // some key
    ),
    //...
),

flutter doctor -v

[✓] Flutter (Channel stable, 3.16.5, on macOS 13.5 22G74 darwin-arm64, locale zh-Hant-TW)
    • Flutter version 3.16.5 on channel stable at /Users/grayhuang/development/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 78666c8dc5 (3 weeks ago), 2023-12-19 16:14:14 -0800
    • Engine revision 3f3e560236
    • Dart version 3.2.3
    • DevTools version 2.28.4

[✓] Android toolchain - develop for Android devices (Android SDK version 33.0.1)
    • Android SDK at /Users/grayhuang/Library/Android/sdk
    • Platform android-33, build-tools 33.0.1
    • Java binary at: /Applications/Android Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.13+0-b1751.21-8125866)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 14.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 14E222b
    • CocoaPods version 1.12.1

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

[✓] Android Studio (version 2021.3)
    • 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 11.0.13+0-b1751.21-8125866)

[✓] VS Code (version 1.85.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.80.0

[✓] Connected device (2 available)
    • macOS (desktop) • macos  • darwin-arm64   • macOS 13.5 22G74 darwin-arm64
    • Chrome (web)    • chrome • web-javascript • Google Chrome 120.0.6099.199

[✓] Network resources
    • All expected network resources are available.

• No issues found!