-----Description-----
Implement a Lean 4 function that merges two ascendingly sorted lists of integers into one single sorted list (ascending). The resulting list must contain all elements from both input lists, preserving their ascending order.

-----Input-----
The input consists of two lists of integers:
arr1: A sorted list of integers (ascending)
arr2: Another sorted list of integers (ascending)

-----Output-----
The output is a list of integers:
Returns a new list containing all elements from arr1 and arr2, sorted in ascending order.

