C#: Migrate extractor to this repository.

csharp/extractor/Semmle.Autobuild/Program.cs

@@ -0,0 +1,28 @@
+using System;
+
+namespace Semmle.Autobuild
+{
+    class Program
+    {
+        static int Main()
+        {
+            var options = new AutobuildOptions();
+            var actions = SystemBuildActions.Instance;
+
+            try
+            {
+                options.ReadEnvironment(actions);
+            }
+            catch (ArgumentOutOfRangeException ex)
+            {
+                Console.WriteLine("The value \"{0}\" for parameter \"{1}\" is invalid", ex.ActualValue, ex.ParamName);
+            }
+
+            var builder = new Autobuilder(actions, options);
+
+            Console.WriteLine($"Semmle autobuilder for {options.Language}");
+
+            return builder.AttemptBuild();
+        }
+    }
+}