Add Windows notifications support urgency/priority levels.
This maps the existing `urgency` option (previously Linux-only) to
Windows toast notification priorities:

- 'critical' maps to ToastNotificationPriority_High, which sorts the
  notification above default-priority items in Action Center.
- 'normal' and 'low' both map to ToastNotificationPriority_Default.

Note that on Windows, 'critical' priority does not prevent the toast
from being auto-dismissed. Users should additionally set `timeoutType`
to 'never' for that behavior.

Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>