Save notebooks with ordered keys

[chrisjsewell]

It would be ideal if the notebook JSON was saved with sorted top-level keys, i.e. ['cells', 'metadata', 'nbformat', 'nbformat_minor']. Currently the order is ['nbformat', 'nbformat_minor', 'metadata', 'cells'].

The reason for this is to reduce git diffs, if for example one also opens and saves the notebook via jupyter notebook or jupyter lab (which save it in this manner).

(as noted in https://github.com/microsoft/vscode-python/issues/10544#issuecomment-598752751)

[DavidKutu]

Good catch @chrisjsewell.

We'll get to it.

[rchiodo]

Validated.