[stream] Make _readableState and _writableState part of the public API
Following up from #7077 (comment), and other discussion.
One of the problem that has lead to breaking userland stream modules is related to the fact that they rely on functionality available in _readableState and _writableState that is not documented, and that is the only possible way for solving some issues, like knowing if a stream is in objectMode: true, clearing up the leftover write callbacks, and many other functionalities.
I propose we document, standardize and test a piece of _readableState and _writableState.
I think we should have been able to spot mafintosh/duplexify#6 before releasing, sigh. @calvinmetcalf @thealphanerd are we setting readable-stream in passthrough mode for citgm run?
I am opening this discussion here rather than in readable-stream because I would like all possible feedback. As @mikeal said, "So, what are we gonna do about streams?" openjs-foundation/summit#16 (comment).
@nodejs/streams we might want probably put this at in our next meeting.