Copilot: Write dnx shim code

Copilot prompt:

Add code to #file:'dnx.cs' to launch a new process.  It should launch the dotnet executable in the same directory as the current program.  It should pass "dnx" as the first argument, and then add all the other arguments that were passed to the current program.

Author: dsplaisted

src/Layout/dnx/dnx.cs

@@ -1,2 +1,59 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Diagnostics;
+using System.IO;
+
+class Program
+{
+    static int Main(string[] args)
+    {
+        // Get the path to the current executable
+        string? exePath = Process.GetCurrentProcess().MainModule?.FileName;
+        if (exePath == null)
+        {
+            Console.Error.WriteLine("Could not determine the path to the current executable.");
+            return 1;
+        }
+        string? exeDir = Path.GetDirectoryName(exePath);
+        if (exeDir == null)
+        {
+            Console.Error.WriteLine("Could not determine the directory of the current executable.");
+            return 1;
+        }
+        // Path to dotnet in the same directory
+        string dotnetPath = Path.Combine(exeDir, "dotnet");
+#if WINDOWS
+        dotnetPath += ".exe";
+#endif
+        // Build argument list: "dnx" + all args
+        string arguments = "dnx";
+        if (args.Length > 0)
+        {
+            arguments += " " + string.Join(" ", args.Select(a => $"\"{a.Replace("\"", "\\\"")}"));
+        }
+        var psi = new ProcessStartInfo
+        {
+            FileName = dotnetPath,
+            Arguments = arguments,
+            UseShellExecute = false
+        };
+        try
+        {
+            using var process = Process.Start(psi);
+            if (process == null)
+            {
+                Console.Error.WriteLine($"Failed to start process: {dotnetPath}");
+                return 1;
+            }
+            process.WaitForExit();
+            return process.ExitCode;
+        }
+        catch (Exception ex)
+        {
+            Console.Error.WriteLine($"Error launching process: {ex.Message}");
+            return 1;
+        }
+    }
+}