Streamline tree API a bit

src/Spectre.Console/Extensions/HasTreeNodeExtensions.cs

@@ -0,0 +1,27 @@
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Contains extension methods for <see cref="IHasCulture"/>.
+    /// </summary>
+    public static class HasTreeNodeExtensions
+    {
+        /// <summary>
+        /// Adds a child to the end of the node's list of children.
+        /// </summary>
+        /// <typeparam name="T">An object type with tree nodes.</typeparam>
+        /// <param name="obj">The object that has tree nodes.</param>
+        /// <param name="child">Child to be added.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static T AddChild<T>(this T obj, TreeNode child)
+            where T : class, IHasTreeNodes
+        {
+            if (obj is null)
+            {
+                throw new System.ArgumentNullException(nameof(obj));
+            }
+
+            obj.Children.Add(child);
+            return obj;
+        }
+    }
+}