feat(solutions): add is-palindrome/cs/function

challenges/is-palindrome/solutions/cs/function/README.md

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+# is-palindrome/cs/function
+
+Created by [@Divlo](https://github.com/Divlo) on 10 September 2021.

challenges/is-palindrome/solutions/cs/function/Solution.cs

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+using System;
+
+namespace Solution
+{
+    class Program
+    {
+        static void Main()
+        {
+            string line = Console.ReadLine();
+            line = line.ToLower().Replace(" ", "");
+            string reverse = "";
+            for (int index = line.Length - 1; index >= 0; index--)
+            {
+                reverse += line[index];
+            }
+            if (line == reverse)
+            {
+                Console.WriteLine("true");
+            }
+            else
+            {
+                Console.WriteLine("false");
+            }
+        }
+    }
+}