Support markdown highlighting in messages

Some plugins like Lazy will send notifications with markdown highlighting and expect the notification backend to render it:


This is straightforward in a plugin like nvim-notify since every notification is its own buffer, but since Fidget renders all notifications into a single buffer, getting Markdown highlighting gets quite a bit trickier..

Start by examining the notification backend and how Fidget renders all notifications into a single buffer, using the contrast with nvim-notify and the Lazy plugin notifications as context. Determine how Markdown highlighting can work without disrupting the shared buffer, and consider the work complete when Markdown-highlighted notifications render correctly alongside other notifications.

Written by the indexing model from the issue text.