coverage: expose native V8 coverage

native V8 coverage reports can now be written to disk by setting the
variable NODE_V8_COVERAGE=dir

PR-URL: https://github.com/nodejs/node/pull/22527


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Yang Guo <yangguo@chromium.org>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Rod Vagg <rod@vagg.org>