Add record for `tuple<stream<u8>, future<result<_, error-code>>>`

[lann]

This type is duplicated between stdout and stderr write-via-stream. I'd like to add some record type to hold these two items and associated documentation for how the two should be used together.

This type pattern also appears in other wasi interfaces (wasi-filesystem, wasi-http, more?), so a goal here would be to establish a convention to help implementers familiarity-wise.