feat(solutions): add acronyms/c/function

challenges/acronyms/solutions/c/function/character.c

@@ -0,0 +1,20 @@
+#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) {
+  char ascii_a = 'a';
+  char ascii_A = 'A';
+  char ascii_z = 'z';
+  if (character >= ascii_a && character <= ascii_z) {
+    return character + (ascii_A - ascii_a);
+  }
+  return character;
+}