Change the type of Diagnostic.message to MarkedString

[smarter]

Currently, diagnostic messages do not carry any formatting information, this forces clients to be conservative when displaying them, see microsoft/vscode#22239 and the proposed solution:

Going forward. Deprecate the current way diagnostics are rendered and make them work just like the regular hover: a Markdown string which defaults to a non-monospace font but still lets language feature providers open up monospace code blocks with triple back quotes.

[dbaeumer]

We could the same that we did for CompletionItems and make this a client capability.

[razzeee]

This is still something that would vastly improve the experience.

[michaelmesser]

Why was this added to on deck then moved to the backlog? Was there some issue? This combined with #1056 would be useful to the Idris 2 LSP.

[dbaeumer]

Wrongly assigned it to On Deck. As you can see changes happened the same day.