icu: make process.binding('icu') internal

PR-URL: https://github.com/nodejs/node/pull/23234
Refs: https://github.com/nodejs/node/issues/22160


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>