ExtensionsInstance must be @ConsumerType since instances of this
interface are provided by the @ConsumerType RuntimeExtension.
Simplify StackedWriter

* Extend from PrintWriter such as to provide PrintWriter API
  StackedWriter consumers in line with the Servlet API
* Remove all overwrites except close which makes sure all entries
  have been pulled off the stack before actually closing
* push() and pop() replace the PrintWriter's out field to replace
  the actual writer to which the PrintWriter sends its output