Add rounded border

src/Spectre.Console/Composition/Border.cs

@@ -17,6 +17,7 @@ namespace Spectre.Console.Composition
             { BorderKind.None, new NoBorder() },
             { BorderKind.Ascii, new AsciiBorder() },
             { BorderKind.Square, new SquareBorder() },
+            { BorderKind.Rounded, new RoundedBorder() },
         };
 
         /// <summary>

src/Spectre.Console/Composition/BorderKind.cs

@@ -19,5 +19,10 @@ namespace Spectre.Console
         /// An old school ASCII border.
         /// </summary>
         Ascii = 2,
+
+        /// <summary>
+        /// A rounded border.
+        /// </summary>
+        Rounded = 3,
     }
 }

src/Spectre.Console/Composition/Borders/RoundedBorder.cs

@@ -0,0 +1,37 @@
+using System;
+
+namespace Spectre.Console.Composition
+{
+    /// <summary>
+    /// Represents a rounded border.
+    /// </summary>
+    public sealed class RoundedBorder : Border
+    {
+        /// <inheritdoc/>
+        protected override string GetBoxPart(BorderPart part)
+        {
+            return part switch
+            {
+                BorderPart.HeaderTopLeft => "╭",
+                BorderPart.HeaderTop => "─",
+                BorderPart.HeaderTopSeparator => "┬",
+                BorderPart.HeaderTopRight => "╮",
+                BorderPart.HeaderLeft => "│",
+                BorderPart.HeaderSeparator => "│",
+                BorderPart.HeaderRight => "│",
+                BorderPart.HeaderBottomLeft => "├",
+                BorderPart.HeaderBottom => "─",
+                BorderPart.HeaderBottomSeparator => "┼",
+                BorderPart.HeaderBottomRight => "┤",
+                BorderPart.CellLeft => "│",
+                BorderPart.CellSeparator => "│",
+                BorderPart.ColumnRight => "│",
+                BorderPart.FooterBottomLeft => "╰",
+                BorderPart.FooterBottom => "─",
+                BorderPart.FooterBottomSeparator => "┴",
+                BorderPart.FooterBottomRight => "╯",
+                _ => throw new InvalidOperationException("Unknown box part."),
+            };
+        }
+    }
+}