This PR updates the assistant to render historical user messages the
same as ones from the assistant.

Double-clicking on a message will open a composer inline for editing.
Pressing `Esc` will cancel the edit.

We don't yet restore the previous state of the message upon canceling.

<img width="401" alt="Screenshot 2024-04-30 at 4 04 01 PM"
src="https://github.com/zed-industries/zed/assets/1486634/5f253fa8-6578-4054-be30-c495e326d700">

<img width="401" alt="Screenshot 2024-04-30 at 4 04 28 PM"
src="https://github.com/zed-industries/zed/assets/1486634/edf25cea-d97e-44d2-8772-3690eac017a4">


Release Notes:

- N/A