stat.rs: Refactor to remove `#[allow(clippy::cognitive_complexity)]`

https://github.com/uutils/coreutils/blob/main/src/uu/stat/src/stat.rs#L407
A potential way to do it:
```
'%' => tokens.append(&mut handle_percent_case(&chars, &mut i, bound, format_str)?),
'\\' => tokens.append(&mut handle_escape_sequences(&chars, &mut i, bound, format_str, use_printf)),
```