feat(#84): add ability to optionally apply abs and negated functions inline

[kylemilloy]

This tickets resolves #84.

Added tests to cover off true/false and closures that return the same. The idea here is that you can optionally assign whether or not you want to make a value absolute or negated based on a logic gate inline. Compare the two code examples of how this is done:

What you would have to do before:

$number = $number->multipliedBy(3)
  ->dividedBy(2);

if ($number->toInt() % 2 === 0) {
  $number = $number->negated();
}

return $number->add(5)->toInt();

And what this allows you to do now:

return $number->multipliedBy(3)
  ->dividedBy(2)
  ->negated(fn (BigNumber $number) => $number->toInt() % 2 === 0)
  ->add(5)
  ->toInt();

or

function getBalance(bool $accountIsCreditNormal) {
    return $number->multipliedBy(3)
      ->dividedBy(2)
      ->negated($accountIsCreditNormal)
      ->add(5)
      ->toInt();
}

Syntax is much more clear and succinct. My use case for this as outlined in #84 is to bring this into brick/money where we require this where we're manipulating values in an accounting system.

[BenMorel]

Closing as per #84.