Add missing finalizer back to BackgroundWorker

The type isn't sealed, it exists in .NET Core purely for backwards
compatibility with existing code, and in the desktop implementation it has
a finalizer.  If an existing component had derived from BackgroundWorker
and overridden Dispose to enable finalization of unmanaged resources, in
.NET Core they would not be cleaned up.  Putting back the missing
finalizer.

Author: jbe2277

src/System.ComponentModel.EventBasedAsync/src/System/ComponentModel/BackgroundWorker.cs

@@ -24,6 +24,11 @@ public BackgroundWorker()
             _progressReporter = new SendOrPostCallback(ProgressReporter);
         }
 
+        ~BackgroundWorker() // exists for backwards compatibility
+        {
+            Dispose(false);
+        }
+
         private void AsyncOperationCompleted(object arg)
         {
             _isRunning = false;