Add missing cell metadata properties

[afeld]

I have a number of notebooks that I've created and maintained through a mix of Jupyter Notebook, Jupyter Lab, and the VSCode Jupyter Extension. I was opening https://github.com/googlecolab/colabtools/issues/5675, and noticed that metadata.slide_type wasn't in the spec. It's used by multiple official Jupyter projects, so seems like it should be…?

In going down that rabbit hole, noticed a few other metadata properties that were missing from the JSONSchema. Added those here as well.

I didn't update the format_description.rst — happy to do so if you like.

Thanks!