Update emoji support

* Add constants for emojis
* Move emoji shortcode rendering to Markup
* Add documentation
* Add example
* Add tests

examples/Emojis/Program.cs

@@ -0,0 +1,17 @@
+using System;
+using Spectre.Console;
+
+namespace Emojis
+{
+    public static class Program
+    {
+        public static void Main(string[] args)
+        {
+            // Markup
+            AnsiConsole.Render(
+                new Panel("[yellow]Hello :globe_showing_europe_africa:![/]")
+                    .RoundedBorder()
+                    .SetHeader("Markup"));
+        }
+    }
+}