Retain selected kernel in untitled notebook when saving notebook

[joyceerhl]

Testing microsoft/vscode-jupyter#10183

1. Create untitled ipynb
2. Run cells
3. ✅ export commands are available in command palette
4. Save the untitled notebook as ipynb and give it a title
5. Open command palette
6. 🐛 all the export commands are now hidden

[joyceerhl]

I think this is because technically the cells were run in the context of the previous notebook, so I now need to run them again after saving the notebook, which is a rather nasty limitation...

[rebornix]

I think this is because after saving we don't have a kernel connected to the newly saved notebook. There is no kernel session transfer (from untitled to saved notebook) currently.

[greazer]

Save As is handled by core. Need to transfer current controller to new ipynb.

[Tyriar]

Seems related to #150890?