Add `variable.special` color to Gruvbox themes (#26271)

This adds the `variable.special` color to the Gruvbox family of themes,
which colors special variables (in Rust's case `self`) differently than
normal ones. The colors were taken from the old Gruvbox `variable`
highlighting (see https://github.com/zed-industries/zed/pull/25464).

before:

![image](https://github.com/user-attachments/assets/3f329ac0-fbdf-480c-9074-5db99591f4e1)

![image](https://github.com/user-attachments/assets/43efdddd-7daf-440f-8c11-d6279330912a)

after:

![image](https://github.com/user-attachments/assets/052c05b8-55c5-495a-a9cc-a5f73aa5aa00)

![image](https://github.com/user-attachments/assets/f598b75f-8d2d-4710-b804-9282de9f8d15)

fixes half of https://github.com/zed-industries/zed/issues/26206. Since
I don't use the Ayu themes I'd prefer someone who knows what looks good
there does those changes.

Release Notes:

- Gruvbox themes: Added a color for `@variable.special` syntax
highlights.