Allow completing '?' to the latest file in a directory

[compholio]

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This completion change allows retrieving the latest file in any _filedir completion (whether it is the current directory or not).

Examples: mv ?<TAB> -> mv latest-file.pdf mv ~/Downloads/?<TAB> -> mv ~/Downloads/latest-file.pdf

[scop]

Let's discuss at https://github.com/scop/bash-completion/discussions/637

This implementation has a few issues that would need fixing before accepting anyway, but I think the time is not right to go that deep into details as there are some larger concerns to resolve first.