Add ido-fallback special variable (Bug#31707)

Add ido-fallback special variable (Bug#31707)

Before ido.el switch to lexical-binding, it was possible for other
packages to modify the 'fallback' variables declared inside
'ido-file-internal' and 'ido-buffer-internal'.
* lisp/ido.el (ido-fallback): New variable.
(ido-buffer-internal, ido-file-internal): Reset ido-fallback to nil
before prompting user.  Use ido-fallback when ido-exit is 'fallback'.
(ido-fallback-command): Add optional FALLBACK-COMMAND argument.

Copyright-paperwork-exempt: yes