lib: refactored scheduling policy assignment

In previous implementation it was clubbed into declaration of scheduling
policies and fetching the schedulingPolicy. Now they are separate
variables, so that in future if one want to add new scheduling policy.
It is much simpler and not obsfucated.

PR-URL: https://github.com/nodejs/node/pull/32663


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>