Raised when the user toggles a task-list checkbox while AreTaskListsInteractive
is true. The checkbox already shows the new state; the viewer does not change
Markdown. SourceStart and
SourceLength locate the item's [ ] / [x] marker
in the rendered markdown (the Markdown string as set, or the text loaded from
Source). Bubbles up the visual tree.
public event EventHandler<TaskToggledEventArgs>? TaskToggled