Enhancement: add sort options for `ls` command

in the past, the output of ls -l was sorted by name. Now it can be sorted by
one of the specifiers (name|size|time=mtime|atime|ctime|extension).
Use --sort <sortable field> to achieve this.

Reverse sorting also has been implemtented. Use --reverse to indicate reverse
sorting.

https://github.com/restic/restic/issues/4179
https://github.com/restic/restic/pull/5182
