Reorder using directives in IAsyncEnumerableExtensions

Rearranged the order of using statements for clarity; no functional changes.

Author: Chris-Wolfgang

src/Wolfgang.Extensions.IAsyncEnumerable/IAsyncEnumerableExtensions.cs

@@ -1,7 +1,7 @@
 using System;
 using System.Collections.Generic;
-using System.Threading;
 using System.Runtime.CompilerServices;
+using System.Threading;
 
 [assembly: InternalsVisibleTo("Wolfgang.Extensions.IAsyncEnumerable.Benchmarks")]
 [assembly: InternalsVisibleTo("Wolfgang.Extensions.IAsyncEnumerable.Tests.Unit")]