This panic was caused because we incorrectly assumed that each character
was one byte when converting character indices to highlight range byte
indices.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #53479

Release Notes:

- Fix a panic that could occur in the project symbol search picker

---------

Co-authored-by: Lukas Wirth <lukas@zed.dev>