fix: improve typing for intersect(), union() and substract()

[nikophil]

here are some typing improvements:

• Intersects should narrow the types:

$intsAndStrings = listOf(['a', 1]); // List<int|string>
$onlyInts = $intsAndStrings->intersect(listOf([1])); // List<int>

• union should widden the types: (actually, I'm wondering if you'll be OK with this one 😅)

$ints = listOf([1, 2]);
$intsAndStrings = $ints->union(listOf(['a']));

• substract should allow anything

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[delacry]

Thank you!

union should widden the types: (actually, I'm wondering if you'll be OK with this one 😅)

Yeah that's okay, type widening is fine as it doesn't widen existing collection, but creates a new one.