Markdown editor for comments (preview)

Introduced a new Markdown editor for work item comments, aligning with the experience in Pull Requests. Existing HTML comments can be optionally converted to Markdown, with a best-effort conversion process. New comments support essential Markdown formats, HTML table conversion, string highlighting via `<mark>`, and continued support for @mentions and !mentions. This feature is exclusively available in the New Boards Hub and will replace the current HTML editor upon general availability.