Event args for a task-list checkbox toggled by the user. Carries the new state and where the
item's [ ] / [x] marker sits in the markdown source, so the host can write the
change back.
public TaskToggledEventArgs(bool isChecked, int sourceStart, int sourceLength, int line)
Raised through TaskToggled, positions refer to the markdown text the viewer rendered: the Markdown string as set, or the document loaded from Source. Raised through TaskToggled, they refer to the text the rendered document was parsed from.