Use native GUI to show child process exited with `wait-after-command`

[mitchellh]

Today, when wait-after-command is set, Ghostty injects that the process has exited directly into the terminal screen, as shown below. This is true on both macOS and in GTK.

Gnome Console uses this native banner instead (below). Every terminal I've tried on macOS does what Ghostty does, surprisingly!

I think this is a lot more pleasant. And using a native widget to highlight information like this is directly in line with Ghostty's philosophy. We should do it, too. This issue is to track that. We should do this on both macOS and GTK.

The exact styling doesn't need to match Gnome Console, and it doesn't need to match across platforms, but it should probably be somewhat similar. We can bikeshed that styling here since it's a detail. There's more work that needs to be done to plumb this information around in core, too.

For styling, I do like the bottom-banner. For copy, I think we should continue to use what we have: "Command exited. Press any key to close the terminal."