Goal: Implement the type version of `Array.join`. `Join<T, U>` takes an array `T`, separator `U` and returns the array `T` with `U` stitching up. Source challenge: https://github.com/type-challenges/type-challenges/tree/main/questions/05310-medium-join Reference explanation: https://github.com/ghaiklor/type-challenges-solutions/blob/main/en/medium-join.md