flattenDeep([1, [2, [3, [4]]]]) // → [1, 2, 3, 4]flattenDeep recursively flattens nested arrays to any depth into a single flat array.
Challenge: Implement flattenDeep(arr: unknown[]): unknown[] flattening arbitrarily nested arrays.
flattenDeep([1, [2, [3, [4]]]]) // → [1, 2, 3, 4]