feat: add doubleTildeToStrikethrough ShortcutEventHandler to built-in ShortcutEvents

frontend/app_flowy/packages/appflowy_editor/lib/src/service/shortcut_event/built_in_shortcut_events.dart

@@ -257,6 +257,11 @@ List<ShortcutEvent> builtInShortcutEvents = [
     command: 'backquote',
     handler: backquoteToCodeHandler,
   ),
+  ShortcutEvent(
+    key: 'Double tilde to strikethrough',
+    command: 'shift+tilde',
+    handler: doubleTildeToStrikethrough,
+  ),
   // https://github.com/flutter/flutter/issues/104944
   // Workaround: Using space editing on the web platform often results in errors,
   //  so adding a shortcut event to handle the space input instead of using the