-----Description----- 
This task requires writing a Lean 4 method that adds two non-empty linked lists representing non-negative integers.
The digits are stored in reverse order (i.e., the first element is the least significant digit).
Each node (list element) holds a single digit (ranging from 0 to 9). The function should add the two numbers and return the sum 
as a linked list, also in reverse order.

-----Input-----
The input consists of:
- l1: A list of natural numbers representing the digits of the first number in reverse order.
- l2: A list of natural numbers representing the digits of the second number in reverse order.

-----Output-----
The output is a list of natural numbers:
Returns a list of digits (in reverse order) representing the sum of the two input numbers.

