-----Description-----
This task requires writing a Lean 4 function that finds the maximum subarray sum from a given list of integers. 
A subarray is a contiguous sequence of elements within the list. 
The function should return the maximum sum that can be obtained from any subarray.

-----Input-----
The input is a list of integers:
xs: A list of integers (can include negative numbers).

-----Output-----
The output is an integer:
Returns the maximum sum among all contiguous subarrays of xs. 
If the list is empty, the result should be 0.

