Enhancement: Add `--ncdu` option to `ls` command

NCDU (NCurses Disk Usage) is a tool to analyse disk usage of directories. It has
an option to save a directory tree and analyse it later.

The `ls` command now supports outputting snapshot information in the NCDU format
using the `--ncdu` option. Example usage: `restic ls latest --ncdu | ncdu -f -`

https://github.com/restic/restic/issues/4549
https://github.com/restic/restic/pull/4550
https://github.com/restic/restic/pull/4911
