Separate output item for stderr?

[sourishkrout]

As follow up for #1763. We still need to resolve.

Btw, this PR does not solve how to include stderr into the serialized notebook, as per:

and for non-interactive we could probably do:

• stateful.runme/output-items
• application/vnd.code.notebook.stdout
• perhaps a third with application/vnd.code.notebook.stderr

Would a separate application/vnd.code.notebook.stderr output item work for you, @jlewi? Alternatively, stderr could be merged into the ``application/vnd.code.notebook.stdout` output item.

[jlewi]

Yes; I think a separate output item would work fine. This would allow Foyle to easily include the stderr in the model prompt.

To consider the alternative, I think one downside of not merging stdout/stderr is that it can be harder to do so after the fact. So if a program is mixing stdout and stderr then errors might miss important contextual information. However, in this case we are talking about non-interactive mode and a motivation for using non-interactive mode (#684) is non-standard renderers. If your using a non-standard renderer then that arguably implies "stdout" and "stderr" should contain different content (e.g. JSON; vs. errors) and shouldn't be merged because that would interfere with the renderer.