Add NotebookCell#index api

[jrieken]

For #105283 we have added operations to replace cells or modify output and metadata for cells. These operations target a cell by notebook-uri and cell index. To make this API easy to consume we should add NotebookCell#index - being the index of the cell in the array of cells.

[jrieken]

We should also consider to remove NotebookCell#uri since it is less important and just a shorthand for NotebookCell#document#uri

[isidorn]

How best to verify this?