- Update diff in highlights.scm to use semantic @diff.plus and
@diff.minus tokens
- Add diff.plus, diff.minus tokens to fallback theme
- Add diff syntax tokens to all official themes (One, Gruvbox, Ayu)

This fixes the issue where diff files showed inverted colors (additions
in purple/blue, deletions in green) by using proper semantic tokens
instead of @string and @keyword.

Related discussion in https://github.com/zed-industries/zed/issues/34057
(thought there might be second issue in that issue)

Release Notes:

- Added support for diff.plus, diff.minus and diff.deleta theme
selectors in the diff syntax grammar. Previously diff.plus was mapped to
@string and diff.minus was mapped to @keyword.

*Before:*

<img width="689" height="566" alt="Screenshot 2025-12-20 at 10 58 34 AM"
src="https://github.com/user-attachments/assets/b7ae4428-2011-4ec2-a803-7e30599e1715"
/>

*After:*

<img width="659" height="548" alt="Screenshot 2025-12-20 at 10 58 30 AM"
src="https://github.com/user-attachments/assets/02df31b5-1f1a-40af-adf0-1ac34457cdd4"
/>

*Before:*

<img width="650" height="545" alt="Screenshot 2025-12-20 at 10 58 16 AM"
src="https://github.com/user-attachments/assets/78b348bd-1b91-478b-a850-8e526adcfb3c"
/>

*After:*

<img width="627" height="563" alt="Screenshot 2025-12-20 at 10 58 21 AM"
src="https://github.com/user-attachments/assets/1176d626-563f-455a-9902-137f646eb8f2"
/>

*Before:*

<img width="628" height="550" alt="Screenshot 2025-12-20 at 10 58 06 AM"
src="https://github.com/user-attachments/assets/d819f478-bb2d-4afd-8013-c4e1a2f64dfe"
/>

*After:*

<img width="643" height="642" alt="Screenshot 2025-12-20 at 10 56 00 AM"
src="https://github.com/user-attachments/assets/e0f03549-77ed-4a47-b1da-239b519b90f4"
/>

---------

Co-authored-by: Finn Evers <finn.evers@outlook.de>
Co-authored-by: MrSubidubi <finn@zed.dev>