outline panel: Add indent guides (#19719)

See #12673

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| <img width="302" alt="image"
src="https://github.com/user-attachments/assets/44b8d5f9-8446-41b5-8c0f-e438050f0ac9">
| <img width="301" alt="image"
src="https://github.com/user-attachments/assets/a2e6f77b-6d3b-4f1c-8fcb-16bd35274807">
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Release Notes:

- Added indent guides to the outline panel