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: nodejs#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>

lib/zlib.js

@@ -786,6 +786,6 @@ const bkeys = Object.keys(constants);
 for (var bk = 0; bk < bkeys.length; bk++) {
   var bkey = bkeys[bk];
   Object.defineProperty(module.exports, bkey, {
-    enumerable: true, value: constants[bkey], writable: false
+    enumerable: false, value: constants[bkey], writable: false
   });
 }