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

challenges/is-palindrome/solutions/c/function/character.c

@@ -0,0 +1,19 @@
+#include "character.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+void character_append(char* string, char character) {
+  size_t length = strlen(string);
+  string[length] = character;
+  string[length + 1] = '\0';
+}
+
+const char character_to_upper(const char character) {
+  int a_ascii_code = (int)'a';
+  int z_ascii_code = (int)'z';
+  if (character >= a_ascii_code && character <= z_ascii_code) {
+    return character - 32;
+  }
+  return character;
+}