Updates Zed2 Copilot UI

- Introduces the `Headline` component for rendering large text
- Styles the 3 Copilot prompt states

![CleanShot 2024-01-08 at 13 19
09@2x](https://github.com/zed-industries/zed/assets/1714999/4b58b85f-23aa-4c22-9ca6-d0abd232cf18)
![CleanShot 2024-01-08 at 13 18
55@2x](https://github.com/zed-industries/zed/assets/1714999/0bf71e1d-63b5-4fbb-8672-e42f8602516d)


Release Notes:

- Updated the connect Copilot modal UI.