Add [DebuggerDisplay] to CancellationTokenSource (dotnet#105764)

* Add [DebuggerDisplay] to CancellationTokenSource

Add `[DebuggerDisplay]` to `CancellationTokenSource` to show whether cancelled or disposed.

Relates to dotnet#105698.

* Update src/libraries/System.Private.CoreLib/src/System/Threading/CancellationTokenSource.cs

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/libraries/System.Private.CoreLib/src/System/Threading/CancellationTokenSource.cs

@@ -22,6 +22,7 @@ namespace System.Threading
     /// concurrently from multiple threads.
     /// </para>
     /// </remarks>
+    [DebuggerDisplay("IsCancellationRequested = {IsCancellationRequested}")]
     public class CancellationTokenSource : IDisposable
     {
         /// <summary>A <see cref="CancellationTokenSource"/> that's already canceled.</summary>