Enable Content-Length header in stdio reader

This change turns on the "Content-Length" header expectation for the
MessageReader that is used in the language service host.  The VS Code
plugin now writes out this header for all of its requests.

Author: daviwil

src/PowerShellEditorServices.Transport.Stdio/StdioHost.cs

@@ -142,7 +142,7 @@ async Task ListenForMessages()
             MessageReader messageReader = 
                 new MessageReader(
                     System.Console.In, 
-                    MessageFormat.WithoutContentLength,
+                    MessageFormat.WithContentLength,
                     messageTypeResolver);
 
             MessageWriter messageWriter =