This PR styles the message input in the chat panel.

<img width="242" alt="Screenshot 2024-01-08 at 4 28 33 PM"
src="https://github.com/zed-industries/zed/assets/1486634/48e83692-878e-4891-8e40-8738cdeb23a8">

Release Notes:

- Improved the styling of the message editor in the chat panel.