This PR is a first pass at styling the welcome screen in Zed2.

Here's the current state:

<img width="1237" alt="Screenshot 2023-12-11 at 7 00 43 PM"
src="https://github.com/zed-industries/zed/assets/1486634/a0cbd5ca-7331-4785-bf46-f83fc4cb3bb6">

Release Notes:

- N/A