bind prints raw control characters when no slash-escape exists for it

[lilyball]

The default set of binds includes a binding on the del control character.

fish-shell/share/functions/fish_default_key_bindings.fish

Line 62 in a4f5dd5

bind --preset $argv \x7f backward-delete-char

But when you ask bind to then print its bindings, it just prints the del control character raw:

bind --preset -k backspace backward-delete-char
bind --preset  backward-delete-char
bind --preset -k sdc backward-delete-char

(piping that into less will show the control character as a highlighted ^?)

bind should instead show any non-printable character as a hex or unicode escape, if there's no key name for it. Though in this particular case maybe it should just show it as \c?, which is a bit confusing and generally can't be typed but is technically accurate.

fish, version 3.1.2

[faho]

bind calls escape_string_script here, which string escape also uses, so that would need to be adjusted.

I'd just make it so DEL ends up being shown as \x7f. That's as likely to be of use as \c?, and is what we actually use in the binding scripts.

I'm not sure if any other characters need this adjustment as well - all control characters?

But since the immediate issue is with DEL, let's do that first.