assistant: Improve JSON handling in `/fetch` command (#12864)

This PR improves the `/fetch` command with better support for URLs that
return JSON content.

JSON response bodies will now be pretty-printed and placed within a
Markdown code block:

<img width="690" alt="Screenshot 2024-06-10 at 3 39 52 PM"
src="https://github.com/zed-industries/zed/assets/1486634/4a7c1cb7-9f5b-4a63-9e8e-5168bf9a6625">

Release Notes:

- Improved the handling of JSON response bodies in the `/fetch` command
in the Assistant.