Add support for hierarchical list prompts

Closes #412

src/Spectre.Console/Widgets/Prompt/List/ListPromptItem.cs

@@ -0,0 +1,80 @@
+using System.Collections.Generic;
+
+namespace Spectre.Console
+{
+    internal sealed class ListPromptItem<T> : IMultiSelectionItem<T>
+        where T : notnull
+    {
+        public T Data { get; }
+        public ListPromptItem<T>? Parent { get; }
+        public List<ListPromptItem<T>> Children { get; }
+        public int Depth { get; }
+        public bool Selected { get; set; }
+
+        public bool IsGroup => Children.Count > 0;
+
+        public ListPromptItem(T data, ListPromptItem<T>? parent = null)
+        {
+            Data = data;
+            Parent = parent;
+            Children = new List<ListPromptItem<T>>();
+            Depth = CalculateDepth(parent);
+        }
+
+        public IMultiSelectionItem<T> Select()
+        {
+            Selected = true;
+            return this;
+        }
+
+        public ISelectionItem<T> AddChild(T item)
+        {
+            var node = new ListPromptItem<T>(item, this);
+            Children.Add(node);
+            return node;
+        }
+
+        public IEnumerable<ListPromptItem<T>> Traverse(bool includeSelf)
+        {
+            var stack = new Stack<ListPromptItem<T>>();
+
+            if (includeSelf)
+            {
+                stack.Push(this);
+            }
+            else
+            {
+                foreach (var child in Children)
+                {
+                    stack.Push(child);
+                }
+            }
+
+            while (stack.Count > 0)
+            {
+                var current = stack.Pop();
+                yield return current;
+
+                if (current.Children.Count > 0)
+                {
+                    foreach (var child in current.Children.ReverseEnumerable())
+                    {
+                        stack.Push(child);
+                    }
+                }
+            }
+        }
+
+        private static int CalculateDepth(ListPromptItem<T>? parent)
+        {
+            var level = 0;
+            while (parent != null)
+            {
+                level++;
+                parent = parent.Parent;
+            }
+
+            return level;
+        }
+    }
+}