cs: add «1800. Maximum Ascending Subarray Sum»

URL:	https://leetcode.com/problems/maximum-ascending-subarray-sum/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/maximum-ascending-subarray-sum.cs

@@ -0,0 +1,15 @@
+public class Solution {
+    private record MaxAscendingAcc(int Sum, int Max) {
+        public MaxAscendingAcc Add(int num) => new(Sum + num, Math.Max(Max, Sum + num));
+        public MaxAscendingAcc Reset(int num) => new(num, Math.Max(Max, num));
+    }
+
+    public int MaxAscendingSum(int[] nums) =>
+        nums.Zip(nums.Skip(1)).Aggregate(
+            new MaxAscendingAcc(nums[0], nums[0]),
+            (acc, pair) => pair switch {
+                _ when pair.First < pair.Second => acc.Add(pair.Second),
+                _ => acc.Reset(pair.Second),
+            }
+        ).Max;
+}