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

~> 1MB | format filesize sec
Error: nu::shell::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