Fix tree rendering

Fixes some tree rendering problems where lines were not properly drawn
at some levels during some circumstances.

* Change the API back to only allow one root.
* Now uses a stack based approach to rendering instead of recursion.
* Removes the need for measuring the whole tree in advance.
  Leave this up to each child to render.

src/Spectre.Console/Rendering/Tree/LineTreeGuide.cs

@@ -0,0 +1,26 @@
+using System;
+
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// A tree guide made up of lines.
+    /// </summary>
+    public sealed class LineTreeGuide : TreeGuide
+    {
+        /// <inheritdoc/>
+        public override TreeGuide? SafeTreeGuide => Ascii;
+
+        /// <inheritdoc/>
+        public override string GetPart(TreeGuidePart part)
+        {
+            return part switch
+            {
+                TreeGuidePart.Space => "    ",
+                TreeGuidePart.Continue => "│   ",
+                TreeGuidePart.Fork => "├── ",
+                TreeGuidePart.End => "└── ",
+                _ => throw new ArgumentOutOfRangeException(nameof(part), part, "Unknown tree part."),
+            };
+        }
+    }
+}