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

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

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+# is-prime-number/c/function
+
+Created by [@Divlo](https://github.com/Divlo) on 29 September 2021.

challenges/is-prime-number/solutions/c/function/solution.c

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+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+bool is_prime_number(int number) {
+  for (int iteration = 2; iteration < number; iteration++) {
+    if (number % iteration == 0) {
+      return false;
+    }
+  }
+  return true;
+}
+
+int main() {
+  int number;
+  scanf("%d", &number);
+  bool is_prime = is_prime_number(number);
+  printf("%s\n", is_prime ? "true" : "false");
+  return EXIT_SUCCESS;
+}