style: (#746)

- convert ValueStringBuilder.cs to file-scoped namespaces
- add rule in .editorconfig

.editorconfig

@@ -3,6 +3,7 @@ root = true
 
 # C# files
 [*.cs]
+csharp_style_namespace_declarations=file_scoped:warning
 
 # Compiler
 dotnet_diagnostic.CS1998.severity = suggestion      # CS1998: Missing awaits

src/Common/CancellableStreamReader/ValueStringBuilder.cs

@@ -8,8 +8,8 @@ using System.Runtime.InteropServices;
 
 #nullable enable
 
-namespace System.Text
-{
+namespace System.Text;
+
 internal ref partial struct ValueStringBuilder
 {
     private char[]? _arrayToReturnToPool;
@@ -314,4 +314,3 @@ namespace System.Text
         }
     }
 }
-}