Added initial support for rendering composites

This is far from complete, but it's a start
and it will enable us to create things like tables
and other complex objects in the long run.

src/Spectre.Console/Composition/IRenderable.cs

@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+using System.Text;
+
+namespace Spectre.Console.Composition
+{
+    /// <summary>
+    /// Represents something that can be rendered to the console.
+    /// </summary>
+    public interface IRenderable
+    {
+        /// <summary>
+        /// Measures the renderable object.
+        /// </summary>
+        /// <param name="encoding">The encoding to use.</param>
+        /// <param name="maxWidth">The maximum allowed width.</param>
+        /// <returns>The width of the object.</returns>
+        int Measure(Encoding encoding, int maxWidth);
+
+        /// <summary>
+        /// Renders the object.
+        /// </summary>
+        /// <param name="encoding">The encoding to use.</param>
+        /// <param name="width">The width of the render area.</param>
+        /// <returns>A collection of segments.</returns>
+        IEnumerable<Segment> Render(Encoding encoding, int width);
+    }
+}

src/Spectre.Console/Composition/Segment.cs

@@ -0,0 +1,130 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using Spectre.Console.Internal;
+
+namespace Spectre.Console.Composition
+{
+    /// <summary>
+    /// Represents a renderable segment.
+    /// </summary>
+    public sealed class Segment
+    {
+        /// <summary>
+        /// Gets the segment text.
+        /// </summary>
+        public string Text { get; }
+
+        /// <summary>
+        /// Gets the appearance of the segment.
+        /// </summary>
+        public Appearance Appearance { get; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Segment"/> class.
+        /// </summary>
+        /// <param name="text">The segment text.</param>
+        public Segment(string text)
+            : this(text, Appearance.Plain)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Segment"/> class.
+        /// </summary>
+        /// <param name="text">The segment text.</param>
+        /// <param name="appearance">The segment appearance.</param>
+        public Segment(string text, Appearance appearance)
+        {
+            Text = text?.NormalizeLineEndings() ?? throw new ArgumentNullException(nameof(text));
+            Appearance = appearance;
+        }
+
+        /// <summary>
+        /// Gets the number of cells that this segment
+        /// occupies in the console.
+        /// </summary>
+        /// <param name="encoding">The encoding to use.</param>
+        /// <returns>The number of cells that this segment occupies in the console.</returns>
+        public int CellLength(Encoding encoding)
+        {
+            return Text.CellLength(encoding);
+        }
+
+        /// <summary>
+        /// Returns a new segment without any trailing line endings.
+        /// </summary>
+        /// <returns>A new segment without any trailing line endings.</returns>
+        public Segment StripLineEndings()
+        {
+            return new Segment(Text.TrimEnd('\n'), Appearance);
+        }
+
+        /// <summary>
+        /// Splits the provided segments into lines.
+        /// </summary>
+        /// <param name="segments">The segments to split.</param>
+        /// <returns>A collection of lines.</returns>
+        public static List<SegmentLine> Split(IEnumerable<Segment> segments)
+        {
+            if (segments is null)
+            {
+                throw new ArgumentNullException(nameof(segments));
+            }
+
+            var lines = new List<SegmentLine>();
+            var line = new SegmentLine();
+
+            foreach (var segment in segments)
+            {
+                if (segment.Text.Contains("\n"))
+                {
+                    if (segment.Text == "\n")
+                    {
+                        lines.Add(line);
+                        line = new SegmentLine();
+                        continue;
+                    }
+
+                    var text = segment.Text;
+                    while (text != null)
+                    {
+                        var parts = text.SplitLines();
+                        if (parts.Length > 0)
+                        {
+                            line.Add(new Segment(parts[0], segment.Appearance));
+                        }
+
+                        if (parts.Length > 1)
+                        {
+                            lines.Add(line);
+                            line = new SegmentLine();
+
+                            text = string.Concat(parts.Skip(1).Take(parts.Length - 1));
+                            if (string.IsNullOrWhiteSpace(text))
+                            {
+                                text = null;
+                            }
+                        }
+                        else
+                        {
+                            text = null;
+                        }
+                    }
+                }
+                else
+                {
+                    line.Add(segment);
+                }
+            }
+
+            if (line.Count > 0)
+            {
+                lines.Add(line);
+            }
+
+            return lines;
+        }
+    }
+}

src/Spectre.Console/Composition/SegmentLine.cs

@@ -0,0 +1,13 @@
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Spectre.Console.Composition
+{
+    /// <summary>
+    /// Represents a line of segments.
+    /// </summary>
+    [SuppressMessage("Naming", "CA1710:Identifiers should have correct suffix")]
+    public sealed class SegmentLine : List<Segment>
+    {
+    }
+}