[markdown] Automatic lists should not add additional completed tasks

Based on feedback in !81079 (comment 859300255)

When adding a new task list item to a list, if the current task item is marked done, [x], then the new one is automatically marked done.

- [x] a completed task
- [x] 

Instead, always create new, uncompleted task

- [x] a completed task
- [ ] 

Also, recognize [X] as a valid task item, like other task handling does