added peek, need testing

include/stack.h

@@ -109,4 +109,14 @@ std::expected<T, StackErr> Stack<T>::pop() {
     return std::expected(return_val);
 }
 
+template <typename T>
+requires std::formattable<T, char>
+std::expected<T, StackErr> Stack<T>::peek() {
+    if (this->len == 0) {
+        return std::unexpected(StackErr::empty);
+    }
+
+    return std::expected(this->data[this->len - 1]);
+}
+
 #endif // !ST