Add new stack data structure

frontend/lib/utils/stack.dart

@@ -0,0 +1,20 @@
+class Stack<T> {
+  final List<T> _list = [];
+
+  void push(T element) {
+    _list.add(element);
+  }
+
+  T pop() {
+    var element = _list.last;
+    _list.removeLast();
+    return element;
+  }
+
+  T peek() {
+    return _list.last;
+  }
+
+  bool get isEmpty => _list.isEmpty;
+  bool get isNotEmpty => _list.isNotEmpty;
+}