Add support for File-> Revert File feature in Custom Editor

[DonJayamanne]

Currently this is't hook up.
Based on docs, we need to revert the Notebook to the originally saved state.

• Today we seem to be attempting to revert it to the last hot exit state
• We aren't modifying the existing model (we're loading a new model)

We'd need to modify the current model and re-send all the cells back up to the UI
Note sure how this impacts undo/redo.
Also not sure how widely this is used.

[DonJayamanne]

FYI - we need to implement the same thing for VSC Notebooks, might be easier there as we don't need to track undo/redo etc, we just need to re-build the VS Code Notebook document anew.

Yay, with VSC Notebooks, our API doesn't need to support this. VSC will just re-open the document.

Hence this feature is only required for Custom editor

[greazer]

Thanks for your feedback. This issue will likely be addressed once VS Code adds native support for editing notebooks. Please see microsoft/vscode#91987.