MietteReporter: Add multiline highlight support

[zkat]

One of the nice things that miette should handle out of the box is having highlights that point to multi-line code blocks.

The protocol as it is right now should be able to handle it just fine, but this seems like just a generally pretty hard thing to write a reporter for, so I'm struggling.

Let me know if you care a lot about this and I'll gladly leave it in your hands, though!