Array unpacking support for string-keyed arrays

[canvural]

PHPStan didn't seem to have support for unpacking string keyed arrays: https://phpstan.org/r/e187af75-320b-4fa6-ba5a-54ceec396f42

This PR adds it only for PHP 8.1 and above.

I think my implementation may not be the best (especially the $hasStringKey hackery), feel free to override it or I'd be happy to change it according to suggestions 👍🏽

[ondrejmirtes]

Hi, I think the else branch is still wrong, right?

} else {
	$arrayBuilder->degradeToGeneralArray();
	$arrayBuilder->setOffsetValueType(new IntegerType(), $valueType->getIterableValueType());
}

[canvural]

Hi, I think the else branch is still wrong, right?

} else {
	$arrayBuilder->degradeToGeneralArray();
	$arrayBuilder->setOffsetValueType(new IntegerType(), $valueType->getIterableValueType());
}

Are you saying this should also apply to the else branch for not ConstantArrayType?

[ondrejmirtes]

Thank you!