util: runtime deprecation for custom .inspect()

Change documentation-only deprecation for custom inspection using
`object.inspect` property to a runtime deprecation.

This is a breaking change. Custom inspection via `object.inspect` is
deprecated because there is a more robust Symbol-based alternative to
`.inspect` and the custom inspection via `object.inspect` feature means
that people can accidentally break `console.log()` simply by attaching a
`.inspect` property to their objects. Note that since this is a
deprecation, the custom inspection will still work. The breaking change
is simply the printing of a warning which could alarm users, break tests
or other things that might be dependent on specific output, etc.

PR-URL: https://github.com/nodejs/node/pull/16393
Ref: https://github.com/nodejs/node/issues/15549


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>