Add waitForExit API to TaskExecution

[ThadHouse]

Currently there is no trivial way to wait for a TaskExecution to be completed. Before 1.44, there was a way to do this using onDidEndTaskProcess, however as of 1.44 this is broken, and a still open bug report (#96643). Instead of needing to do workarounds with callbacks and condition variables to wait for a task to complete, a much simpler solution would be to add a waitForExit(cancel: CancellationToken): Thenable<number> api directly to TaskExecution. This would allow an easy way to wait for a process to exit, without needing to go through callback nightmares, especially when those callbacks break.

[meganrogge]

This should not be needed when #96643 is fixed