Add netstandard2.0 support (#33)

* Add netstandard2.0 support

src/Common/Polyfills/System/Threading/CancellationTokenSourceExtensions.cs

@@ -0,0 +1,17 @@
+using System.Text;
+
+namespace System.Threading.Tasks;
+
+internal static class CancellationTokenSourceExtensions
+{
+    public static Task CancelAsync(this CancellationTokenSource cancellationTokenSource)
+    {
+        if (cancellationTokenSource is null)
+        {
+            throw new ArgumentNullException(nameof(cancellationTokenSource));
+        }
+
+        cancellationTokenSource.Cancel();
+        return Task.CompletedTask;
+    }
+}