`code` directive's `emphasize-lines` parameter doesn't work without `linenos` enabled

[mfisher87]

Description

I'm working on a tutorial for JupyterCon to build an extension from scratch, step-by-step. When we have users modify their code, I want to highlight what's changed. I don't want to display line numbers, because the reader could think that the line numbers should reflect the line numbers they see in their editor and be briefly confused. It's extraneous and distracting information, and I feel a good tutorial should not have any of that :)

I could use :lineno-start: as a workaround for the potential confusion, but it's a pain to keep that updated, and I worry I'll make mistakes and that'll be even more confusing 😆

Proposed solution

I don't know what the underlying cause is so I don't have much to propose :)

Additional notes

Noted in https://github.com/jupyter-book/mystmd/issues/1809