Add scrollbar to Extensions view

[zisra]

Summary

Zed Extension view does not have a scrollbar and should have one.

Environment

Zed: v0.131.6 (Zed)
OS: macOS 14.4.1
Memory: 16 GiB
Architecture: aarch64

If applicable, add mockups / screenshots to help explain present your vision of the feature

[zisra]

Still reproducible.