Refactor backend code related to notebooks

[greazer]

• Unwrap baseJupyterSession wrapper layers (INotebook and friends`)
• Transition use of executeObservable in nativeInteractiveWindow to new kernelExecution.ts codepath
• Remove dependence on ICell interface. We should directly deal with nbformat cells and notebooks
• Add findMatchingWindow(uri: Uri) to IInteractiveWindowProvider and cleanup nativeInteractiveWindowCommandListener
• One unified command handler for kernel restart and interrupt, shared across native notebooks and interactive window

Here's a set of issues from early 2020 that may or may not related to this work item. Feel free to just close if no longer applicable.

• Remove our old webPanelServer code #1595
• Split the INotebook into smaller types #1603
• NativeEditor.ts and InteractiveWindow.ts could have their 'execution' logic split off #1606

[DonJayamanne]

Discussion points:

• Interactive window should use Native Notebook Kernel provider & kernel execution for running cells
  Completed by @joyceerhl
• Remove notion of Restarting Kernels & replace with Stop & Start new kernel
  • Today we have some specific code that deals with restarting kernels & re-initializing the kernel once it has been restarted. In reality the restart is identical to start, we end up with a brand new kernel and we need to run the initialization code again.

[greazer]

Closing. Much of this has been done or changed to the point that this issue no longer applies well. Using #8933 to track this type of work.