Closes#14469
# Description
- ~~Implement the ``--unit`` conversion in "into int" command~~
- New ``ShellError::InvalidUnit`` unit if users enter wrong units
- Made ``ShellError::CantConvertToDuration`` more generic: became
``CantConvertToUnit``
- Tried to improve the way we parse units and get the supported units.
It's not complete, though, I will continue this refactoring in another
PR. But I already did some small refactorings in the "format duration"
and "format filesize" commands
- Add tests for "format filesize" and "format duration"
# User-Facing Changes
```nu
~> 1MB | format filesize sec
Error: nu:🐚:invalid_unit
× Invalid unit
╭─[entry #7:1:23]
1 │ 1MB | format filesize sec
· ─┬─
· ╰── encountered here
╰────
help: Supported units are: B, kB, MB, GB, TB, PB, EB, KiB, MiB, GiB, TiB, PiB, EiB
```
