tui: highlight "skip mode" tasks

[oliver-sanders]

The Tui equivalent to cylc/cylc-ui#1949

Find some way to make "skip mode" tasks stand out. We could follow suit with the UI and use a task modifier, however, we have fewer pixels to play with in Tui, it might be difficult to find a unicode modifier that makes sense and formats with single character width in all commonly used terminals.

At the very least we can cram it into the task summary.