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 googlecolab/colabtools#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!

[rgbkrk]

Each of these new optional boolean fields need to have "default": true since the absence of these fields would likely be interpreted as false. That or it ends up in ambiguous territory even though we'd probably want to assume these are true in notebooks.

Something like:

"editable": {
  "description": "Whether the cell is editable.",
  "type": "boolean",
  "default": true
},

for each