[vscode] Stub onWillSaveNotebookDocument (1.78 new API)

[rschnekenbu]

What it does

Stub vscode 'Notebook' onWillSaveNotebookDocument & NotebookDocumentWillSaveEvent API
This enables the possibility to load and start plugins which depend on the new 1.78 'Notebook' API on workspace.

Fixes #12529

Contributed on behalf of ST Microelectronics

How to test

1. Install following extension:

• extension: notebook-onwillsave-extension-0.0.1.zip
• source: notebook-onwillsave-extension-src.zip

2. Reproduce the problem in the theia instance before switching to this PR. When starting theia with the extension installed, an error will pop-up telling that Activating extension 'notebook-onwillsave-extension' failed: o.workspace.onWillSaveNotebookDocument is not a function
3. Update your theia repository to use this PR
4. Open the same workspace and make sure the message above is no longer displayed. Nothing should happen, as the API is only stubbed, not implemented.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

LGTM 👍