Full strikethrough of a task list item

Description

Following discussions on an issue, the following situation is often interesting to keep tabs of evolving requirements without necessarily removing task items:

- [X] this task is done
- [ ] this task is pending
- [ ] ~~this task has been deemed useless~~

Yet the task item is counted towards the total as well as being clickable. You could click it but it's not really "done" so that's a bit weird.

Proposal

Support a task list item that can have a strikethrough, rendering a checkbox but disabling it in the UI as well as leaving it out of task item counting.

Proposed syntax:

- [ ] ~~this task has been deemed useless~~
- ~~[ ] this task has been deemed useless~~
- [~] this task has been deemed useless

The first one is natural and currently renders a checkbox but feels like some "lookahead". The second one hierarchically makes sense but currently does not render a checkbox as well as not having the ASCII boxes aligned, which just feels like bad UX. The third one seems nice enough.