Rename the [Ee]xts dir to ext (#38852)

**WARNING:** This PR will probably blow up all in-flight PRs

at some point in the early days of CmdPal, two of us created seperate
`Exts` and `exts` dirs. Depending on what the casing was on the branch
that you checked one of those out from, it'd get stuck like that on your
PC forever.

Windows didn't care, so we never noticed.

But GitHub does care, and now browsing the source on GitHub is basically
impossible.

Closes #38081

src/modules/cmdpal/ext/SamplePagesExtension/Program.cs

@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading;
+using Microsoft.CommandPalette.Extensions;
+
+namespace SamplePagesExtension;
+
+public class Program
+{
+    [MTAThread]
+    public static void Main(string[] args)
+    {
+        if (args.Length > 0 && args[0] == "-RegisterProcessAsComServer")
+        {
+            using ExtensionServer server = new();
+            var extensionDisposedEvent = new ManualResetEvent(false);
+            var extensionInstance = new SampleExtension(extensionDisposedEvent);
+
+            // We are instantiating an extension instance once above, and returning it every time the callback in RegisterExtension below is called.
+            // This makes sure that only one instance of SampleExtension is alive, which is returned every time the host asks for the IExtension object.
+            // If you want to instantiate a new instance each time the host asks, create the new instance inside the delegate.
+            server.RegisterExtension(() => extensionInstance);
+
+            // This will make the main thread wait until the event is signalled by the extension class.
+            // Since we have single instance of the extension object, we exit as soon as it is disposed.
+            extensionDisposedEvent.WaitOne();
+        }
+        else
+        {
+            Console.WriteLine("Not being launched as a Extension... exiting.");
+        }
+    }
+}