Use built in ipynb serializer to convert NotebookData to ipynb

[DonJayamanne]

This prevents having to carry ICell around,
as we end up having to expose Jupyter cell types around.
Simpler and better to use the NoetbookCellData instead

For microsoft/vscode#205637

[DonJayamanne]

Consider using an extension point from ipynb provider,
We already have a methdo exportNotebook, created 3 years ago, we shoudl use that
That would result in less code duplication for genrating ipynb from a list of Cells.