Support Application.ActiveWindow.SelectedSheets

Blocks feature parity with our legacy COM add-in. We have a functionality "process selected worksheets" but Office.js does not expose this API.

We have customers who set up data uploads in a workbook and only want to process some of them using our add-in. For that purpose, they select several (but not all) worksheets and click the "Process" button in our add-in.

Code snippet from COM-add-in:

Sheets selectedSheets = application.ActiveWindow.SelectedSheets;