Resolve "Add is empty function"

Closes #10 (closed)

stack.c

 #include <stdlib.h>
+#include <stdbool.h>
 #include "stack.h"
 
 
@@ -75,3 +76,13 @@ int get_length(stack s) {
     return s.top + 1;
 }
 
+/**
+ * @brief Check if a stack is empty
+ * 
+ * @param s
+ * @return bool - true if stack is empty, false otherwise
+ */
+bool stack_is_empty(stack s)
+{
+    return s.top == -1;
+}
\ No newline at end of file