Extensibility API: Add ability to execute initialization code when a kernel is started

[pwang347]

Motivation

It would be very useful to allow extensions to run initialization code whenever a kernel is started (or restarted). This would allow extensions to create richer notebook experiences (e.g., modifying the environment or exposing custom modules/helpers/variables) that are immediately available when a user executes a cell for the first time. One particularly interesting application of this is allowing extensions to register IPython formatters at the same time as contributing a VS Code Notebook Renderer to complete the full end-to-end story of providing richer outputs with a simple versioning story with no external package dependencies.

API Proposal

See https://github.com/microsoft/vscode-jupyter/blob/main/src/api.proposed.kernelStartHook.d.ts

Progress

• Add an API to listen to kernel start events - Add public API event for kernel post-initialization #16214
• Allow event callers to defer queueing of user cells until initialization completes - Add waitUntil for onDidStart event #16375
• API finalization

CC @DonJayamanne @rebornix