Add support for readonly Notebook Cell

[DonJayamanne]

Jupyter supports the concept of marking a notebook cell as readonly and not deletable see here https://nbformat.readthedocs.io/en/latest/format_description.html#cell-metadata

Suggestion: Can we have support for marking a cell as readonly and not deletable as per the nbotebook file format spec

[rebornix]

We used to have this at the very beginning and then later on made way for Serializer API, with which the readonliness is controlled by the underling resource but not metadata. TextDocument in cells inherit the readonliness from the notebook document.

Now that the metadata is simply a dictionary I wonder if we would support this in the same ways as Collapse. cc @roblourens

[vscodenpa]

We closed this issue because we don't plan to address it in the foreseeable future. If you disagree and feel that this issue is crucial: we are happy to listen and to reconsider.

If you wonder what we are up to, please see our roadmap and issue reporting guidelines.

Thanks for your understanding, and happy coding!

[FerdinandoR]

This would really be useful, I always am afraid of re-editing cells that I don't want to change by mistake and occasionally do so. Please reconsider :)