Support Mermaid graphs in Chat output

[digitarald]

Give Copilot a whiteboard.

For when a question needs a visual, especially when Copilot Chat explains or plans complex systems.

Would need a system prompt to tell it when to use visuals to explain, and a way to preview to render mermaid (easy to style and LLMs know the format well).

[iwangbowen]

We need this feature

[torsknod2]

@digitarald Mermaid is for sure a good start, but I am not sure whether it is sufficient. I guess you will also need PlantUML, which to my (lack of) experience seems to support a lot more diagram types.

[digitarald]

First parts landed. We also got a first cut at extension surfaces for chat output, so PlantUML could use that eventually.