stream: split `objectMode` for Duplex

This commit introduces `readableObjectMode` and
`writableObjectMode` options for Duplex streams.
This can be used mostly to make parsers and
serializers with Transform streams.

Also the docs section about stream state objects
is removed, because it is not relevant anymore.
The example from the section is remade to show
new options.

fixes #6284

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>