adapted namespaces for MemoryExtension Split methods also

Author: dragon7307

src/Extensions/ReadOnlySpan/Span/Split.cs

@@ -1,10 +1,11 @@
-using System.Buffers;
+using System;
+using System.Buffers;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 
 #if !NET9_0_OR_GREATER
 
-namespace System
+namespace SpanExtensions
 {
 #pragma warning disable CS1591 // Missing XML comment for publicly visible type or member
     public static partial class MemoryExtensions

src/Extensions/Span/Span/Split.cs

@@ -1,10 +1,11 @@
-using System.Buffers;
+using System;
+using System.Buffers;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 
 #if !NET9_0_OR_GREATER
 
-namespace System
+namespace SpanExtensions
 {
 #pragma warning disable CS1591 // Missing XML comment for publicly visible type or member
     public static partial class MemoryExtensions