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?

この issue では、ファイル、テスト、または特定のエントリポイントが特定されていません。まずターミナル pager の実装を見つけて #46 の議論を読み、そのうえで cerr の扱いについてプロジェクトが受け入れる方針を明確にします。完了とは、その決定に従って cerr が処理され、pager の作業を妨げない状態です。