Streamline tree API a bit

src/Spectre.Console/IHasTreeNodes.cs

@@ -0,0 +1,15 @@
+using System.Collections.Generic;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Represents something that has tree nodes.
+    /// </summary>
+    public interface IHasTreeNodes
+    {
+        /// <summary>
+        /// Gets the children of this node.
+        /// </summary>
+        public List<TreeNode> Children { get; }
+    }
+}