Add notebookKernel contextkey

[gjsjohnmurray]

This PR allows microsoft/vscode-jupyter#9037 to be fixed.

It adds a notebookKernel contextkey (string type) which will contain the id of the selected kernel.

Kernel id syntax is publisher.name/notebookControllerId

notebookControllerId is the identifier string passed in the first parameter of the vscode.notebooks.createNotebookController method.

The new contextkey can then be used in the when clause of an extension's contribution to the interactive/toolbar menu (for example) to prevent an extension's commands from displaying when the interactive window is using a kernel contributed by a different extension.

For example, extension ms-toolsai.jupyter would add:

  "when": "notebookKernel =~ /^ms-toolsai.jupyter\//"

to its contributions to the interactive/toolbar menu.

pinging @rchiodo

[gjsjohnmurray]

pinging @rebornix

[rebornix]

This is a good approach for limiting actions to notebook using kernels a specific extension contributes. @gjsjohnmurray thanks for your contribution, it looks great!

[rebornix]

We might want to change the context keys to notebookController* later, currently we call them all notebookKernel*. cc @roblourens