feat(solutions): add `is-prime-number/cs/function

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challenges/is-prime-number/solutions/cs/function/README.md

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

challenges/is-prime-number/solutions/cs/function/Solution.cs

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+using System;
+using System.Collections.Generic;
+
+namespace Solution
+{
+    class Program
+    {
+        static void Main()
+        {
+            int number = int.Parse(Console.ReadLine());
+            if (IsPrime(number))
+            {
+                Console.WriteLine("true");
+            }
+            else
+            {
+                Console.WriteLine("false");
+            }
+        }
+
+        /// <summary>
+        /// Checks if a number is prime.
+        /// </summary>
+        /// <param name="number">The number to check.</param>
+        /// <returns>True if the number is prime, false otherwise.</returns>
+        static public bool IsPrime(int number)
+        {
+            return GetDividers(number).Length == 2;
+        }
+
+        /// <summary>
+        /// Gets the dividers of a number.
+        /// </summary>
+        /// <param name="number">The number to get the dividers of.</param>
+        /// <returns>An array of the dividers of the number.</returns>
+        static public int[] GetDividers(int number)
+        {
+            List<int> dividers = new List<int>();
+            for (int index = 1; index <= number; index++)
+            {
+                if (number % index == 0)
+                {
+                    dividers.Add(index);
+                }
+            }
+            return dividers.ToArray();
+        }
+    }
+}