Support formatting of entire ipynb notebook on save

[minsa110]

Feedback from PyCon 2022

"It'd be awesome to have automatic Black formatting on save, like Python does! I can format each cell right now, but I want to be able to format the entire notebook on a command, for example, on notebook save."

Also (somewhat) related to:

• Format cell on run like the nb_black extension does. vscode-jupyter#7058

[greazer]

We believe support for formatting notebooks with Black exists. However, there's still a need to support a "format_on_save" event from the VS Code api to accomplish this.

[rebornix]

From tomorrow's Insiders, you should be able to format on save by turning on "notebook.formatOnSave.enabled": true. We are looking into supporting format modified ranges (if you use SCM) soon.