Support readline shortcuts ctrl-n and ctrl-p as aliases for up and down

[jmcphers]

Discussed in #6635

^{Originally posted by wurli March 5, 2025}
Fairly straightforward :) For some reason I just can't get used to the arrow keys for this, and I don't think it's possible to add this as a user keymap.

Thank you!

These are two specific readline shortcuts that don't have implementations. See #1953 for more general readline keybinding discussion.