child_process: move anonymous class to top level

Move the anonymous class out of setupChannel to clarify code.

PR-URL: https://github.com/nodejs/node/pull/11147


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>