Enhancement: Add `sort` option to `ls` command

The `ls -l` command output can now be sorted using the new `--sort <field>`
option for the fields `name`, `size`, `time` (same as `mtime`), `mtime`,
`atime`, `ctime` and `extension`. A `--reverse` option is also available.

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