fish_key_reader: Humanize key descriptions

This used to print all codepoints outside of the ASCII range (i.e.
above 0x80) in \uXXXX or \UYYYYYYYY notation.

That's quite awkward, considering that this is about keys that are
being pressed, and many keyboards have actual symbols for these on
them - I have an "ö" key, so I would like to use `bind ö` and not
`bind \u00F6`. So we go by iswgraph.

On a slightly different note, `\e` was written as `\c[ (or \e)`. I do
not believe anyone really uses `\c[` (the `[` would need to
be escaped!), and it's confusing and unnecessary to even mention that.

(cherry picked from commit 55c425a0dd)

doc_src/cmds/fish_key_reader.rst

@@ -56,7 +56,7 @@ Example
    > fish_key_reader --verbose
    Press a key:
    # press alt+enter
-              hex:   1B  char: \c[  (or \e)
+              hex:   1B  char: \e
    (  0.027 ms)  hex:    D  char: \cM  (or \r)
    bind \e\r 'do something'