Function pushIfNotIn

Pushes all the items passed to the first array if it doesn't already contain them.

This does NOT check the array for duplicates.

This is because we can just pass an empty array as the first argument to achieve the same thing if we want this to behave like a union (though union is available as a wrapper), while not losing any performance when we're sure there are no duplicates in the first array (e.g. we got it from Object.keys).

const arr1 = ["a", "b", "b"]
const arr2 = ["c", "c"]
const arrUnion = pushIfNotIn(arr1, arr2)
// or pushIfNotIn(arr1, ["c", "c"])
// arrUnion = arr1
// arr1/arrUnion = ["a", "b", "b", "c"]

// or to use like unique union
const arr1 = ["a", "b", "b"]
const arr2 = ["c", "c"]
const arrUnion = pushIfNotIn([], arr1, arr2)
// arrUnion = ["a", "b", "c"]
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