feat: truncate to length + tests

[atymic]

I'll add docs after your review :)

[chrisminett]

Thanks @atymic ! I'll take a proper look in the next week. First thoughts (may be some wrong assumptions here from a quick glance!):

• Glad to see it keeps immutability
• I agree with the changes to "expectedException", but that's out-of-scope for this change.
  • Current code is for PHP 5.6+, but I've been going through all Phlib repos in the past few weeks to make them ^7.3|^8.0 at a minimum, which includes upgrading PHPUnit to v9 and making this exception change amongst others. I'll do this repo next, then this PR could be rebased from that.
• I wonder about keeping the truncate behaviour in a separate class, so that the existing length class can just be about the inspection. It is useful that the truncate() method returns a new instance which will have all the same inspection methods on it though, so I'm not sure what the wiring would look like for that.

[chrisminett]

Closes #4

[atymic]

See #9 same situ as other PR, sorry.