prototypes to create new cells: in-between, + style, dropdown

[Light2Dark]

📝 Summary

• subtler left bar to create new cells (create above opens a dropdown, create below is as normal)
• in between cells a few buttons appear

https://github.com/user-attachments/assets/203498da-e5b4-4512-a8f8-5c94695ebe69

🔍 Description of Changes

📋 Checklist

• [ ] I have read the contributor guidelines.
• [ ] For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• [ ] I have added tests for the changes made.
• [ ] I have run the code and verified that it works as expected.