Add Two Numbers - Sum of Two Linked List Numbers

You are given two non-empty linked lists, l1 and l2, where each represents a non-negative integer with its digits stored in reverse order (e.g., the number 342 is represented as 2 -> 4 -> 3).

Your task is to add the two numbers represented by l1 and l2 and return their sum as a new linked list, with its digits also stored in reverse order.

You may assume that:

• Each node in the input linked lists contains a single digit.
• The input linked lists will not have any leading zeros, with the sole exception being the number 0 itself (which is represented by a single node containing 0).

Return the head of the new linked list representing the sum.