AST: Code-cell should be turned into a block with output

[rowanc1]

There is some discussion and notes here:
https://github.com/orgs/executablebooks/discussions/1037

I think this can be done in the code-cell directive (here).

Changing the return of that to a [{type: 'block', children: [code, {type: output, data: []}]] will I think fix the issue. We should make sure this matches the notebook output and have a few tests around this for consistency!