Communicate cell labels to Jupyter kernel.

[allefeld]

I suggested in #10541 that knitr's behavior, to replace code of the form <<label>> with the code from the referenced cell, for Jupyter kernels. @cscheid replied that this is unlikely to be implemented, and should better be implemented on the side of the kernel.

But for this to be possible, the kernel needs to be informed about the label (if any) of each sent code block.

[cscheid]

I don't think Jupyter's notebook execution model includes this information.

So we either:

• successfully petition the Jupyter folks to add this to a future version of Jupyter (but that's a huge, slow-moving ship - it'll take a long time)
• define a Quarto-specific sideband signaling protocol.

Let's assume the latter for now. Jupyter does allow this through its comms: https://jupyter-client.readthedocs.io/en/latest/messaging.html#comm-info

Quarto already uses this in notebook.py:

https://github.com/quarto-dev/quarto-cli/blob/04524462d5653b57969db2daa49a0b4dfc13bc19/src/resources/jupyter/notebook.py#L213-L219

We could implement this by sending additional per-cell messages in the quarto_comm channel (is that what they call this?).

[allefeld]

Sounds good!

But, this is something-that-would-be-neat-to-have-sometime, not important.