Use native VS Code api instead of old Loggers to detect cell execution

[DonJayamanne]

Today we have nativePostExecute
We added this specifically for the new Debugger coee.
I think this should be removed & we should just use native VS Code API to detect whena cell has been executed.

export class DebuggingCellMap implements IDebuggingCellMap, INotebookExecutionLogger {

This class introduced a new event into a logger that basically wraps existing VSCode API.