-----Description-----
Implement the insertion sort algorithm in Lean 4. The function takes a single list of integers
as input and returns a new list that contains the same integers in ascending order.

Implementation must follow a standard insertion sort approach, placing each element into its correct position.
The resulting list must be sorted in ascending order.
The returned list must be a permutation of the input list (i.e., contain exactly the same elements).

-----Input-----
A single list of integers, denoted as xs.

-----Output-----
A list of integers, sorted in ascending order.

Example:
Input:  [3, 1, 4, 2]
Output: [1, 2, 3, 4]

