Add tree widget

src/Spectre.Console.Tests/Expectations/TreeRenderingTests.Representative_Tree.verified.txt

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+Root node
+├── child1
+│   ├── multiple 
+│   │    line 0
+│   ├── multiple 
+│   │    line 1
+│   ├── multiple 
+│   │    line 2
+│   ├── multiple 
+│   │    line 3
+│   ├── multiple 
+│   │    line 4
+│   ├── multiple 
+│   │    line 5
+│   ├── multiple 
+│   │    line 6
+│   ├── multiple 
+│   │    line 7
+│   ├── multiple 
+│   │    line 8
+│   └── multiple 
+│        line 9
+├── child2
+│   └── child2Child
+│       └── Child 2 child
+│            child
+└── child3
+    └── single leaf
+         multiline
+        └──                2020 January                
+            ┌─────┬─────┬─────┬─────┬─────┬─────┬─────┐
+            │ Sun │ Mon │ Tue │ Wed │ Thu │ Fri │ Sat │
+            ├─────┼─────┼─────┼─────┼─────┼─────┼─────┤
+            │     │     │     │ 1   │ 2   │ 3   │ 4   │
+            │ 5   │ 6   │ 7   │ 8   │ 9   │ 10  │ 11  │
+            │ 12  │ 13  │ 14  │ 15  │ 16  │ 17  │ 18  │
+            │ 19  │ 20  │ 21  │ 22  │ 23  │ 24  │ 25  │
+            │ 26  │ 27  │ 28  │ 29  │ 30  │ 31  │     │
+            │     │     │     │     │     │     │     │
+            └─────┴─────┴─────┴─────┴─────┴─────┴─────┘

src/Spectre.Console.Tests/Expectations/TreeRenderingTests.Root_Node_Only.verified.txt

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+Root node

src/Spectre.Console.Tests/Unit/TreeMeasureTests.cs

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+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using Spectre.Console.Rendering;
+using Xunit;
+
+namespace Spectre.Console.Tests.Unit
+{
+    public class TreeMeasureTests
+    {
+        [Fact]
+        public void Measure_Tree_Dominated_Width()
+        {
+            // Given
+            var nestedChildren =
+                Enumerable.Range(0, 10)
+                    .Select(x => new TreeNode(new Text($"multiple \n line {x}")));
+            var child3 = new TreeNode(new Text("child3"));
+            child3.AddChild(new TreeNode(new Text("single leaf\n multiline")));
+            var children = new List<TreeNode>
+            {
+                new(new Text("child1"), nestedChildren), new(new Text("child2")), child3,
+            };
+            var root = new TreeNode(new Text("Root node"), children);
+            var tree = new Tree(root);
+
+            // When
+            var measurement = ((IRenderable)tree).Measure(new RenderContext(Encoding.Unicode, false), 80);
+
+            // Then
+            // Corresponds to "│   └── multiple"
+            Assert.Equal(17, measurement.Min);
+
+            // Corresponds to "    └── single leaf" when untrimmed
+            Assert.Equal(19, measurement.Max);
+        }
+
+        [Fact]
+        public void Measure_Max_Width_Bound()
+        {
+            // Given
+            var root = new TreeNode(new Text("Root node"));
+            var currentNode = root;
+            foreach (var i in Enumerable.Range(0, 100))
+            {
+                var newNode = new TreeNode(new Text(string.Empty));
+                currentNode.AddChild(newNode);
+                currentNode = newNode;
+            }
+
+            var tree = new Tree(root);
+
+            // When
+            var measurement = ((IRenderable)tree).Measure(new RenderContext(Encoding.Unicode, false), 80);
+
+            // Then
+            // Each node depth contributes 4 characters, so 100 node depth -> 400 character min width
+            Assert.Equal(400, measurement.Min);
+
+            // Successfully capped at 80 terminal width
+            Assert.Equal(80, measurement.Max);
+        }
+
+        [Fact]
+        public void Measure_Leaf_Dominated_Width()
+        {
+            // Given
+            var root = new TreeNode(new Text("Root node"));
+            var currentNode = root;
+            foreach (var i in Enumerable.Range(0, 10))
+            {
+                var newNode = new TreeNode(new Text(string.Empty));
+                currentNode.AddChild(newNode);
+                currentNode = newNode;
+            }
+
+            var tree = new Tree(root);
+
+            // When
+            var measurement = ((IRenderable)tree).Measure(new RenderContext(Encoding.Unicode, false), 80);
+
+            // Then
+            // Corresponds to "│   │   │   │   │   │   │   │   │   └── "
+            Assert.Equal(40, measurement.Min);
+
+            // Corresponds to "│   │   │   │   │   │   │   │   │   └── "
+            Assert.Equal(40, measurement.Max);
+        }
+    }
+}

src/Spectre.Console.Tests/Unit/TreeRenderingTests.cs

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+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Spectre.Console.Testing;
+using VerifyXunit;
+using Xunit;
+
+namespace Spectre.Console.Tests.Unit
+{
+    [UsesVerify]
+    public sealed class TreeRenderingTests
+    {
+        [Fact]
+        public Task Representative_Tree()
+        {
+            // Given
+            var console = new FakeConsole(width: 80);
+            var nestedChildren =
+                Enumerable.Range(0, 10)
+                    .Select(x => new TreeNode(new Text($"multiple \n line {x}")));
+            var child2 = new TreeNode(new Text("child2"));
+            var child2Child = new TreeNode(new Text("child2Child"));
+            child2.AddChild(child2Child);
+            child2Child.AddChild(new TreeNode(new Text("Child 2 child\n child")));
+            var child3 = new TreeNode(new Text("child3"));
+            var child3Child = new TreeNode(new Text("single leaf\n multiline"));
+            child3Child.AddChild(new TreeNode(new Calendar(2020, 01)));
+            child3.AddChild(child3Child);
+            var children = new List<TreeNode> { new(new Text("child1"), nestedChildren), child2, child3 };
+            var root = new TreeNode(new Text("Root node"), children);
+            var tree = new Tree(root);
+
+            // When
+            console.Render(tree);
+
+            // Then
+            return Verifier.Verify(console.Output);
+        }
+
+        [Fact]
+        public Task Root_Node_Only()
+        {
+            // Given
+            var console = new FakeConsole(width: 80);
+            var root = new TreeNode(new Text("Root node"), Enumerable.Empty<TreeNode>());
+            var tree = new Tree(root);
+
+            // When
+            console.Render(tree);
+
+            // Then
+            return Verifier.Verify(console.Output);
+        }
+    }
+}