Review API JupyterServerProvider and prepare for stable API

[rebornix]

This would be our exercise for our first stable API JupyterServerProvider

Tasks

Beta Give feedback

1. Use CancellationError and return undefined to distinguish between cancel and back (see how done in VS code Kernels)

[DonJayamanne]

This is where the discussions are taking place #14106

[DonJayamanne]

API Discussion outcomes

• Use CancellationError and return undefined to distinguish between cancel and back (see how done in VS code Kernels)
• Commands is tricky
  • We would like to enforce a good UX (ensure 3rd party extension have back button etc)
  • The command API is very much tied to the UI

[DonJayamanne]

Done