attach req as res.req

[ianstormtaylor]

This change adds a res.req property which refers to the original IncomingMessage request object.

The reasons for this small change are explained in #28673, but a quick summary:

• All existing Node HTTP frameworks do this under the covers when wrapping http.createServer.
• Referencing the original request is required to handle certain HTTP edge cases when writing responses.
• Node's internals already reference the req in multiple places when determining response-specific behavior.
• Retrieving the res from the req is already possible, this just adds the reverse.
• This would give userland the ability to easily handle these valid HTTP-related edge cases.
• This would allow for intuitive Lodash-like HTTP helper libraries, instead of relying on monolithic Express-like frameworks.

I'd recommend reading #28673 in full, I put a lot of thought/research into the different arguments.

You can imagine a world where you aren't forced to use frameworks for HTTP in Node, and instead we might benefit from a Lodash-like utility library with smaller, functional utilities…

…this would be helpful not only to those who don't want to use frameworks, but also those attempting to building framework-independent libraries and utilities, which is currently almost impossible in the Node ecosystem.

Thanks!

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Also, to be clear, I'm not wed to the res.req naming scheme if it needs to be changed for any reason. As long as there's a way to access the original request I don't care what the way is.

Checklist

• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines