process: make `exitCode` configurable again
This change was done in #44711, and it was unintentional. It caused #45683, and also makes it impossible to mock out the exitCode in tests. It affects all of node 19, and all of node 20 thus far (so it should not be backported to node 18 or earlier)
Filing this PR per https://github.com/nodejs/node/pull/44711#discussion_r1320660607
Refs: https://github.com/nodejs/node/pull/44711 Fixes: https://github.com/nodejs/node/issues/45683