querystring: move isHexTable to internal

PR-URL: https://github.com/nodejs/node/pull/11858


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>