Message view display improvements

[mchakravarty]

• [ ] Underline character at indicated column.
• [ ] Perform highlighting also in the minimap.

[austincondiff]

There are a few other things I've noticed that I've also mentioned in #43.

• [ ] Typography needs to be more consistent
• [ ] Border radius too big
• [ ] More consistent message type icon
• [ ] Icon for each error in message detail popover
• [ ] Background colors / Vibrancy effect
• [ ] Animation on enter, exit, and click
• [ ] Dismiss popover button needs to be on the right

[mchakravarty]

@austincondiff Thanks for the additional items. Some of these points I simply didn't cover in the first cut (such as vibrancy, animations, and the dismiss button) and I definitely want to fix those.

With some of the others (border radius and icons), I didn't intend to exactly replicate what Xcode is doing. I also don't mind to replicate it, but I did those popups in SwiftUI and it wasn't clear to me how to get the exact same visuals as Xcode. I am not sure how important it is to you, for it to look exactly like Xcode or just be in the same style as Xcode. In any case, I would welcome concrete suggestions as to how to change the SwiftUI views rendering the popups for those issues.