Allow sorting of files/dirs ignoring casing

### Description

You can see in this screenshot that directories that start with a lowercase letter are sorted alphabetically, accounting for casing. There should be a flag that lets you ignore casing.

This list should be something like this:

```
A...
a...
B...
b...
C...
...
and so on.
```

- Relevant Issues : (none)
- Relevant PRs : (none)
- Type of issue :
- [ ] Installation
- [ ] Font-related
- [x] Feature request
- [ ] Bug in existing feature
- [ ] Developer mode : Code quality / Tests / Documentation

Start by locating the directory-listing sorting logic and the command-line option definitions in colorls. Add the requested opt-in casing behavior and verify that entries are ordered case-insensitively when the flag is used, while existing ordering remains unchanged.

Written by the indexing model from the issue text.