Implemented merge sort

[KausLol]

Description

Implemented the merge() method in the Sorter class using merge sort algorithm.

Fixes #3

Changes Made

• Implemented Sorter.merge() method with divide-and-conquer algorithm
• Added merge_helper() function to merge sorted sublists
• Added merge_sort_recursive() function for recursive sorting
• Ensured immutability so the original list is never modified

Test Results

All 12 merge sort tests passing (100%)

• Basic sorting (ascending/descending)
• Edge cases (empty, single element, duplicates, negatives, floats)
• Immutability verification
• Performance test (100,000 elements in < 0.7s)
• Custom objects with comparators
• Already sorted and reverse sorted lists

[lindelwa122]

@KausLol, impressive work 👏