src: add not-weak DCHECK to PersistentToLocal::Strong

Refs: https://github.com/nodejs/node/pull/38821#issuecomment-851644135

PR-URL: https://github.com/nodejs/node/pull/38875


Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>