util: add private fields preview support

util.previewValue may reveal private fields of an object.

Fixes: https://github.com/nodejs/node/issues/27404

Checklist

Feature list:

• reveal primitive values
• reveal nested objects with prototypes
• reveal properties of nested objects, not supported
• reveal function names

Common list:

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines