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

challenges/sorting-algorithms/solutions/c/bubble-sort/solution.c

@@ -17,7 +17,7 @@ int main() {
     printf("%d\n", numbers[index]);
   }
   free(numbers);
-  return 0;
+  return EXIT_SUCCESS;
 }
 
 void bubble_sort(int numbers[], const int length) {

challenges/sorting-algorithms/solutions/c/insertion-sort/solution.c

@@ -17,7 +17,7 @@ int main() {
     printf("%d\n", numbers[index]);
   }
   free(numbers);
-  return 0;
+  return EXIT_SUCCESS;
 }
 
 void insertion_sort(int numbers[], const int length) {

challenges/sorting-algorithms/solutions/c/merge-sort/solution.c

@@ -17,7 +17,7 @@ int main() {
     printf("%d\n", numbers[index]);
   }
   free(numbers);
-  return 0;
+  return EXIT_SUCCESS;
 }
 
 void merge(int numbers[], int left_index, int middle_index, int right_index) {