Expose the active notebook document to stable API

[DonJayamanne]

As an extension author I cannot access the active notebook document today.

• Stable API doesn't expose the activeNotebook document

E.g. if i were to create a command for Notebooks, this command is useless as I don't know what the active notebook is (the only way around this is to contribute editor toolbar icons that will pass the Uri in the command handler).

[vscode-triage-bot]

Pinging notebook topic followers: @claudiaregio @rchiodo @greazer @DonJayamanne @jilljac @IanMatthewHuff @tanhakabir @dynamicwebpaige

[jrieken]

/duplicate of #125758

[vscode-triage-bot]

Thanks for creating this issue! We figured it's covering the same as another one we already have. Thus, we closed this one as a duplicate. You can search for existing issues here. See also our issue reporting guidelines.

Happy Coding!