Make Markdown table sorting generally available

What does this MR do and why?

Prepares general availability of sortable Markdown tables.

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

1. Open a rendered Markdown table in an MR diff comment.
2. Confirm gon.features has no markdownSortableTableColumns property.
3. Cycle through ascending, descending, and reset using the sort button.
4. Confirm the original row order returns and tooltips describe the next action.
5. Check a one-row or over-1,000-row table has no sorting controls.

MR acceptance checklist

Evaluated against the MR acceptance checklist.