`ScrollMark(Up|Down)` should scroll to absolute beginning/end if it would overflow

[whisperity]

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Trivial alternative to #836.

Abstract

Executing ScrollMarkUp to jump to the previous prompt should jump to the current beginning of the scrollback buffer in case the previous prompt no longer exists. If we have two large outputs in the buffer with only one prompt in the middle, we need to press ScrollToTop explicitly, instead of ScrollMarkUp falling back to its behaviour.

In an analogous fashion, if the last jumped marker is the last prompt and the "next" does not exist yet (because the process is still running, perhaps), then ScrollMarkDown should behave the same way as ScrollToBottom

Motivation

C++ SFINAE outputs.

[christianparpart]

Lovely motivation! :)