Replaces emoji regex with ReadOnlySpan implementation

The RegEx runtime perf was never anything noticeable - it was the startup time that was eating over a third of time during initialization.

This shaves 200ms off the startup time.

src/Spectre.Console/Extensions/StringBuilderExtensions.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Globalization;
 using System.Text;
 
@@ -25,5 +26,16 @@ namespace Spectre.Console
 
             return builder.Append(value);
         }
+
+        public static void AppendSpan(this StringBuilder builder, ReadOnlySpan<char> span)
+        {
+            // NetStandard 2 lacks the override for StringBuilder to add the span. We'll need to convert the span
+            // to a string for it, but for .NET 5.0 we'll use the override.
+#if NETSTANDARD2_0
+            builder.Append(span.ToString());
+#else
+            builder.Append(span);
+#endif
+        }
     }
 }