Language Model Tool for commenting in a multibuffer (#11509)

Language Model can now open multibuffers and insert comments as block
decorations in the editor.


![image](https://github.com/zed-industries/zed/assets/836375/f4456ad0-66e7-4ad6-a2b3-63810a3223a5)


Release Notes:

- N/A

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Co-authored-by: max <max@zed.dev>
Co-authored-by: marshall <marshall@zed.dev>
Co-authored-by: nate <nate@zed.dev>