zlib: make “bare” constants un-enumerable

We prefer for users to use `zlib.constants.XXX` instead of `zlib.XXX`.
Having both enumerable means that inspecting the `zlib` module
shows both variants, making the output significantly longer and
redundant.

PR-URL: https://github.com/nodejs/node/pull/24824


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>