Support markdown highlighting in messages

[j-hui]

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..

[andysazima]

@j-hui do you have an example of how this can be used to render markdown in a fidget notify message?