Formatting: Add formatList(arr) func to get string of list items

[Farkhanda-Dalal]

Description:
The formatList(arr) function will format an array of strings into a human-readable list.

Acceptance Criteria:

["apples","bananas","cherries"] → "apples, bananas, and cherries"

["apples","bananas"] → "apples and bananas"

["apple"] → "apple"

[] → "" (empty string for empty array)

The function must support proper Oxford comma handling.
Add comprehensive unit tests for arrays of different lengths (0, 1, 2, many).