41. Flatten Nested Arrays to Single Level

Implement a function that flattens a deeply nested array structure into a single-level array.

Your function should:

• Handle arrays nested to any depth
• Preserve the order of elements
• Handle non-array values correctly
• Work with mixed types (numbers, strings, objects, etc.)

Example

flatten([1, [2, [3, [4]], 5]])
// Output: [1, 2, 3, 4, 5]

flatten([[1, 2], [3, 4], [5]])
// Output: [1, 2, 3, 4, 5]

flatten([1, [], [2, [[[3]]]]])
// Output: [1, 2, 3]