zlib: split JS code as prep for non-zlib-backed streams

Split the `Zlib` class into `ZlibBase` and `Zlib` classes,
to facilitate introduction of similar streams with minor
implementation differences.

Backport-PR-URL: https://github.com/nodejs/node/pull/25228
PR-URL: https://github.com/nodejs/node/pull/24939


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>