enhancement: implement deepDuplicateArray (closes #28)

[dwp23]

Description

This PR implements the deepDuplicateArray method in the helpful.js library.
The method allows creating a deep copy of an array, including all nested subarrays, which solves the issue of modifying nested elements affecting the original array.

Issue

Closes #28 – Add deepDuplicateArray method.

Solution

• Uses the existing duplicateArray method for a shallow copy.
• Recursively duplicates any subarrays within the array.
• Returns a fully independent copy of the original array.

Example

let testArray = [1, 2, [3, 4], 5];
let duplicatedArray = helpful.deepDuplicateArray(testArray);

duplicatedArray[2][0] = 99;
console.log(testArray);       // [1, 2, [3, 4], 5]
console.log(duplicatedArray); // [1, 2, [99, 4], 5]

Notes

• Passing null to the function returns an empty array [].
• Works recursively for arrays of any depth.