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?

[roblourens]

Not sure @Tyriar - we are handling this the same way we persist selection and other view state. But the editor still gets reopened with a new model. Dunno how this concept works in custom editors.

[Tyriar]

Custom editors have one the untitled editor closed and a new one with another URI open, I want a signal that a custom editor can handle to upgrade it so no close/open needs to happen

[roblourens]

Steps:

• Open an untitled jupyter notebook
• Select a kernel
• Save the notebook
• The same kernel should be selected

The "Set kernel as default" gear icon in the kernel picker was also removed.

[jrieken]

Verified with this "kernel"

const ctrl = vscode.notebooks.createNotebookController('fff', 'jupyter-notebook', 'FOO');
	ctrl.executeHandler = (cells) => {
		for (let cell of cells) {
			const exec = ctrl.createNotebookCellExecution(cell)
			exec.start()
			exec.replaceOutput(new vscode.NotebookCellOutput([vscode.NotebookCellOutputItem.text(`hello@${Math.random()}`)]))
			exec.end(true)
		}
	}