stream: add flatMap to readable

דארןלק<The normal caveat of this shouldn't landing until https://github.com/nodejs/node/pull/41573 does and being a draft until then and to only look at the relevant commit :)

I am thinking about changing the toArray() behavior to not flatten a non-object-mode stream since I think things get tricky when flatMaping binary and non-binary streams together but I am honestly not sure and want feedback on much of the behaviours here.

Like the other iterator helpers PRs, this involves work co-authored by @ronag

cc @nodejs/streams

I am mostly interested in feedback about what the semantics should be and how flatMap works when flatMapping one stream mode and returning the other type from the mapper.