direnv's .envrc files are just Bash scripts.

---

Self-Review Checklist:

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

Release Notes:

- Detect .envrc files as Shell Script

Co-authored-by: Yara 🏳️‍⚧️ <git@yara.blue>