Add `NotificationQueue` component

[metonym]

Another basic functionality for a component library: a component that manages and instruments a notification queue.

The motivation is to replace the HeaderWithToast.svelte example in UI Shell, and support this at the library level.

https://github.com/carbon-design-system/carbon-components-svelte/blob/master/docs/src/pages/components/UIShell.svx#L92-L96

Component API:

• It should use ToastNotification and allow consumers to imperatively add and remove notification elements.
• Each element can be keyed, so that duplicate notifications are not appended. Auto-dismiss/customizable timeouts should be forwarded to ToastNotification.
• It should support top right and bottom right placements. For top-right, it should sit below the shell. The consumer should be able to customize the margin offsets for either position).

Related #1644

[metonym]

Released in v0.94.0