util: show External values explicitly in inspect

Display `v8::External` values as `[External]` rather than `{}`
which makes them look like objects.

PR-URL: https://github.com/nodejs/node/pull/12151


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>