Change the default staging and unstaging state display (#26299)

This adds a setting for the "border" hunk display mode, as discussed,
and makes it the default.

Here's how it looks in light mode:

<img width="1512" alt="Screenshot 2025-03-07 at 11 39 25 AM"
src="https://github.com/user-attachments/assets/a934faa3-ec69-47e1-ad46-535e48b98e9f"
/>

And dark mode: 

<img width="1511" alt="Screenshot 2025-03-07 at 11 39 56 AM"
src="https://github.com/user-attachments/assets/43c9afd1-22bb-4bd8-96ce-82702a6cbc80"
/>


Release Notes:

- Git Beta: Adjusted the default hunk styling for staged and unstaged
changes

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Nate <nate@zed.dev>