Handle `cerr` in terminal pager

Need to handle cerr as well as cout in terminal pager. Discussion from #46:

Ian:
We do need a strategy for what to do with cerr, but I don't know what is best yet.

Currently cerr will be written to the normal terminal buffer, so it won't be seen whilst the alternative buffer is being used but it reappears when the alternative buffer is disabled. This is perhaps not a good solution, but I don't think it is too bad for a first implementation. Some other options:

1. Capture it and sent it to the same place as cout, it will appear in the output flow in the alternative buffer. We'd probably need to colour it to make is easily visible as it could be anywhere.
2. Capture it and display it at the bottom of the alternative buffer for better visibility, but then there might be quite a lot of information displayed.
3. Capture it and if anything at all is written to it we could avoid using the pager at all, just display the errors in the normal terminal flow.

Johan:
I have a preference for option 1, but I think we can discuss it in a dedicated issue and solve it in a dedicated PR so that it does not block this one, WDYT?

Das Issue nennt keine Dateien, Tests oder einen spezifischen Einstiegspunkt. Beginne damit, die Implementierung des Terminal-Pagers zu finden und die Diskussion aus #46 zu lesen, und stelle dann fest, welche Strategie für den Umgang mit cerr das Projekt akzeptiert. Erledigt ist die Aufgabe, wenn cerr entsprechend dieser Entscheidung behandelt wird, ohne die Arbeit am Pager zu blockieren.