feat: permissions only as numbers

while the usual display of permissions as `.rw-r-----` is nice and dandy,
for some it might be shorter and clearer to see the actual number.

`.rw-r-----` => `0640`

That would be nice feature to have a concise and easy to read display.
And there already is

```
eza -la --octal-permissions
0640 .rw-r----- 128 root 10 Mar 15:30 foo.env
```

But why does it print both? I only want to see the octal permissions.

Start by tracing the existing `eza -la --octal-permissions` option through the CLI handling and output formatting. Confirm how the current combined symbolic and octal output is produced, then define completion as a way to display only octal permissions while preserving the existing option behavior and coverage.

Written by the indexing model from the issue text.