Include C# textmate grammar. Fixes #19469

extensions/csharp/test/colorize-fixtures/test.cs

@@ -0,0 +1,17 @@
+using System;
+namespace SampleNamespace
+{
+    class TestClass
+    {
+        static void Main(string[] args)
+        {
+            int[] radii = { 15, 32, 108, 74, 9 };
+            const double pi = 3.14159;
+            foreach (int radius in radii) {
+                double circumference = pi * (2 * radius);
+                // Display the number of command line arguments:
+                System.Console.WriteLine("Circumference = {0:N2}", circumference);
+            }
+        }
+    }
+}