This allows scripts to make the user choose from a list of entries by
typing part of their text and/or by navigating them with keybindings,
like dmenu or fzf.
Closes#13964.
When running the console in the terminal, style log lines with the same
escape sequences as msg.c.
mp.input can also specify terminal escape sequences, e.g. a script to
select a playlist entry can invert the color of the selection.
Also add a missing newline to help's error message.