Style notifications for notification panel (#4163)

This PR styles the notifications for the notification panel.

These are the notification toasts you receive when you have a new
notification (e.g., a mention in chat):

<img width="423" alt="Screenshot 2024-01-19 at 11 39 16 AM"
src="https://github.com/zed-industries/zed/assets/1486634/a1fc7a67-5aad-4cb2-8dc9-35d302984fcb">

Release Notes:

- Improved the look of toasts for incoming notifications.