assert: lazy load acorn

This makes sure acorn is only loaded in case it is necessary.

PR-URL: https://github.com/nodejs/node/pull/19863


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>