Sticky multibuffer headers (#22391)

https://github.com/user-attachments/assets/92cc5ff7-d8be-4e4b-ac6e-68eb310fffce

Release Notes:

- Multibuffer headers will now stick to the top of the viewport as you
scroll
- Added support for expanding diagnostic excerpts

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Co-authored-by: Michael <michael@zed.dev>