Add more borders for grids, tables, and panels

* Ascii2
* AsciiDoubleHead
* Double
* DoubleEdge
* Heavy
* HeavyEdge
* HeavyHead
* Horizontal
* Minimal
* MinimalDoubleHead
* MinimalHeavyHead
* Simple
* SimpleHeavy

src/Spectre.Console/Rendering/Borders/MinimalBorder.cs

@@ -0,0 +1,37 @@
+using System;
+
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// Represents a minimal border.
+    /// </summary>
+    public sealed class MinimalBorder : 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.CellRight => " ",
+                BorderPart.FooterBottomLeft => " ",
+                BorderPart.FooterBottom => " ",
+                BorderPart.FooterBottomSeparator => " ",
+                BorderPart.FooterBottomRight => " ",
+                _ => throw new InvalidOperationException("Unknown box part."),
+            };
+        }
+    }
+}