benchmark: spread operator benchmark

Benchmark comparing `util._extend()`, `Object.assign()`,
and the spread operator for object assignment.

`util._extend()` still wins currently.

PR-URL: https://github.com/nodejs/node/pull/18442


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>