Show and allow editing of typeahead

# Description of the new feature/enhancement

While a command is running today, you can be typing the next command(s), but you can't see what you're typing.
It would be wonderful if for example a horizontal splitter showed up at the bottom of the screen,
above which would be the console output of the currently running command,
and below which would be a standard editing line.
Type more than one command, and the splitter moves up to make room.

The issue names no files, tests, or entry points, so first clarify the scope with maintainers before locating the relevant terminal input and command-execution code. Done should mean that input typed while a command runs is visible and editable, with the splitter making room for multiple queued commands.

Written by the indexing model from the issue text.