Notification for when kernel is ready

[JasonWeill]

Problem

There is no notification when a kernel has started. A new user may be unfamiliar with the iconography of the kernel UI.

This is a follow up to #15421, which pops up a toast notification when the user tries to run a cell while the kernel is initializing.

Proposed Solution

Pop up a toast notification when the kernel has finished initializing.

Additional context

Let users disable future notifications, such as with an action button to never show kernel notifications again, and a settings option to toggle them.