This PR fixes the sizing of the workspace, specifically with regards to
the center pane.

This fixes the issue where the tab bar would get clipped when its width
exceeded the size of the screen.

<img width="1298" alt="Screenshot 2023-12-12 at 8 36 15 PM"
src="https://github.com/zed-industries/zed/assets/1486634/592d7c6d-6901-4bd4-b5e7-e30bcad67e21">

Release Notes:

- N/A