Add checklist count to work item description

Opportunity

In legacy issues we show the count of checklist items below the description. For parity, and especially to support description truncation, we should add the checklist count in the same format to the work items description.

Acceptance criteria

Exactly the same UX as on legacy issues.

• Checklist count appears when checklist items are present, showing "<countCompleted> of <countTotal> checklist items completed"
• Checklist count updates automatically on check
• Checklist counts updated when description changed resulting in different checklist items
• Shows a ✔ when all items are completed

👇 see checklist item design in this legacy issue