Enhancement: Add `--human-readable` option to `ls` and `find` commands

Previously, when using the `-l` option with the `ls` and `find` commands, the
displayed size was always in bytes, without an option for a more human readable
format such as MiB or GiB.

The new `--human-readable` option will convert longer size values into more 
human friendly values with an appropriate suffix depending on the output size.
For example, a size of `14680064` will be shown as `14.000 MiB`.

https://github.com/restic/restic/issues/4159
https://github.com/restic/restic/pull/4351
