Draft: Parse checkboxes in Markdown tables

What does this MR do and why?

This will fix Parse checkboxes in Markdown tables (#21506) when done.

So far, we have rudimentary support: they render, and in work item description/note views, they correctly toggle on click!

* [x] ok
* [ ] ok!
* [~] ok!?

| hmm | wonderful |
| :-: | :-: |
| [x] | [ ] |
| [~] | [x] |

Okay!

Still to do:

• Support in Taskable (note "1 of 2 checklist items complete" in the above screenshot, and no system notes are generated currently)
  • Blocked by Remove deckar01-task_list (!214761).
• Support in TaskListToggleService, and whatever frontend components support that (for e.g. MRs and other non-work item places)
• Rich text editor support
• Decide what to do about the corresponding text: there's not a clear option for their aria-label, or for the system note body when these are toggled
  • a11y-wise, using the column header isn't a terrible idea ("Check option in column 'wonderful'"), since the row context is relatively easy to navigate to.
  • This doesn't help us for the system note. It could be as simple as concatenating the text content of the cells in the row with separators.
• más?

Screenshots or screen recordings

Before	After

How to set up and validate locally

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