improve handling of the escape character

Increase the delay between seeing an escape character and giving up on
whether additional characters that match a key binding are seen. I'm
setting the value to 500 ms to match the readline library. We don't need
such a large window for sequences transmitted by a terminal (even over ssh
where network delays can be a problem).  However, we can't expect humans to
reliably press the escape key followed by another key with an inter-char
delay of less than ~250 ms based on my testing and research. A value of
500 ms provides a nice experience even for people using "fish_vi_mode"
bindings as a half second to switch from insert to normal mode is still
fast enough that most people won't even notice.

Resolves #1356

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