Add rule widget

Adds a new rule widget.
Also fixes some bugs I encountered while testing
some unrelated things in an extremely small console.

src/Spectre.Console/Widgets/Rule.cs

@@ -0,0 +1,132 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Spectre.Console.Internal;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// A renderable horizontal rule.
+    /// </summary>
+    public sealed class Rule : Renderable, IAlignable
+    {
+        /// <summary>
+        /// Gets or sets the rule title markup text.
+        /// </summary>
+        public string? Title { get; set; }
+
+        /// <summary>
+        /// Gets or sets the rule style.
+        /// </summary>
+        public Style? Style { get; set; }
+
+        /// <summary>
+        /// Gets or sets the rule's title alignment.
+        /// </summary>
+        public Justify? Alignment { get; set; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Rule"/> class.
+        /// </summary>
+        public Rule()
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Rule"/> class.
+        /// </summary>
+        /// <param name="title">The rule title markup text.</param>
+        public Rule(string title)
+        {
+            Title = title ?? throw new ArgumentNullException(nameof(title));
+        }
+
+        /// <inheritdoc/>
+        protected override IEnumerable<Segment> Render(RenderContext context, int maxWidth)
+        {
+            if (Title == null || maxWidth <= 6)
+            {
+                return GetLineWithoutTitle(maxWidth);
+            }
+
+            // Get the title and make sure it fits.
+            var title = GetTitleSegments(context, Title, maxWidth - 6);
+            if (Segment.CellLength(context, title) > maxWidth - 6)
+            {
+                // Truncate the title
+                title = Segment.TruncateWithEllipsis(title, context, maxWidth - 6);
+                if (!title.Any())
+                {
+                    // We couldn't fit the title at all.
+                    return GetLineWithoutTitle(maxWidth);
+                }
+            }
+
+            var (left, right) = GetLineSegments(context, maxWidth, title);
+
+            var segments = new List<Segment>();
+            segments.Add(left);
+            segments.AddRange(title);
+            segments.Add(right);
+            segments.Add(Segment.LineBreak);
+
+            return segments;
+        }
+
+        private IEnumerable<Segment> GetLineWithoutTitle(int maxWidth)
+        {
+            var text = new string('─', maxWidth);
+            return new[]
+            {
+                new Segment(text, Style ?? Style.Plain),
+                Segment.LineBreak,
+            };
+        }
+
+        private (Segment Left, Segment Right) GetLineSegments(RenderContext context, int maxWidth, IEnumerable<Segment> title)
+        {
+            var alignment = Alignment ?? Justify.Center;
+
+            var titleLength = Segment.CellLength(context, title);
+
+            if (alignment == Justify.Left)
+            {
+                var left = new Segment(new string('─', 2) + " ", Style ?? Style.Plain);
+
+                var rightLength = maxWidth - titleLength - left.CellLength(context) - 1;
+                var right = new Segment(" " + new string('─', rightLength), Style ?? Style.Plain);
+
+                return (left, right);
+            }
+            else if (alignment == Justify.Center)
+            {
+                var leftLength = ((maxWidth - titleLength) / 2) - 1;
+                var left = new Segment(new string('─', leftLength) + " ", Style ?? Style.Plain);
+
+                var rightLength = maxWidth - titleLength - left.CellLength(context) - 1;
+                var right = new Segment(" " + new string('─', rightLength), Style ?? Style.Plain);
+
+                return (left, right);
+            }
+            else if (alignment == Justify.Right)
+            {
+                var right = new Segment(" " + new string('─', 2), Style ?? Style.Plain);
+
+                var leftLength = maxWidth - titleLength - right.CellLength(context) - 1;
+                var left = new Segment(new string('─', leftLength) + " ", Style ?? Style.Plain);
+
+                return (left, right);
+            }
+
+            throw new NotSupportedException("Unsupported alignment.");
+        }
+
+        private IEnumerable<Segment> GetTitleSegments(RenderContext context, string title, int width)
+        {
+            title = title.NormalizeLineEndings().Replace("\n", " ").Trim();
+            var markup = new Markup(title, Style);
+            return ((IRenderable)markup).Render(context.WithSingleLine(), width - 6);
+        }
+    }
+}