Add waitUntil for onDidStart event (#16375)

* WIP

* WIP

* Update

* logging WIP

* clean

* Fix tests

* tests

* PR

* more tests

* fix tests

* PR

* fix test

* fix test

* fix test

* Minor fixes

* Updated with a few changes

* Revert "Updated with a few changes"

This reverts commit 797958d.

* Fire when restarting the kernel

---------

Co-authored-by: Don Jayamanne <don.jayamanne@outlook.com>

Author: pwang347

src/api.proposed.kernelStartHook.d.ts

@@ -1,7 +1,7 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
-import type { Event, Uri } from 'vscode';
+import type { CancellationToken, Event, Uri } from 'vscode';
 
 declare module './api' {
     export interface Kernels {
@@ -11,6 +11,16 @@ declare module './api' {
         onDidStart: Event<{
             uri: Uri;
             kernel: Kernel;
+            token: CancellationToken;
+            /**
+             * Allows to pause the event loop until the provided thenable resolved.
+             * This can be useful to ensure startup code is executed before user code.
+             *
+             * *Note:* This function can only be called during event dispatch.
+             *
+             * @param thenable A thenable that delays kernel startup.
+             */
+            waitUntil(thenable: Thenable<unknown>): void;
         }>;
     }
 }