Enhancement: Add file mode in symbolic notation to `ls --json`

Now `restic ls --json` provides mode in symbolic notation aligned
with `restic find --json`.

https://github.com/restic/restic/issues/3542
https://github.com/restic/restic/pull/3573
https://forum.restic.net/t/restic-ls-understanding-file-mode-with-json/4371