doc: add an example for util.types.isExternal

added usage example for util.types.isExternal
which was missing owing to the complexity.
Used a combination of n-api and js to demonstrate
usage of the api.

PR-URL: https://github.com/nodejs/node/pull/31173
Fixes: https://github.com/nodejs/node/issues/20604


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>