Issue #297 - Add markdown rendering to MLX Chat Example

[JoeJoe1313]

Changes done

• Adopt MarkdownUI
• Apply markdown only to assistant message
• Introduce a toggle to choose between plain text and markdown - this is applied both during and after output generation

This addresses #297.

Future improvements

• Apply markdown to user message (but I find this meaningful only after the prompt can support multiline input)

Testing done

[davidkoski]

@JoeJoe1313 see discussion in #297 -- I think we should go with the approach from #303, but thank you so much for working on this!