util: group array elements together

When using `util.inspect()` with `compact` mode set to a number, all
array entries exceeding 6 are going to be grouped together into
logical parts.

PR-URL: https://github.com/nodejs/node/pull/26269


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>