Fix diff highlights (#38384)

Per
#23371 (reply in thread),
the issue is not new and I don't know how to solve the problem more
holistically yet.

All of the native themes don't have spec for `@diff.plus` and
`@diff.minus` leaving addition and deletion not being highlighted. For
diff file, the most valuable highlighting comes from exactly what we're
missing. Hence, I think this is worth fixing.

Perhaps, the ideal fix would be standardizing and documenting captures
such as `@diff.plus` and `@diff.minus` on
https://zed.dev/docs/extensions/languages#syntax-highlighting for theme
writers to adopt. But the existing list of captures seems to be
language-agnostic so I'm not sure if that's the best way forward.

Per
tree-sitter-grammars/tree-sitter-diff#18 (comment),
`tree-sitter-diff`'s author prefers using `@keyword` and `@string` so
that `tree-sitter highlight` can work out of the box. So it seems to be
an ok choice for Zed.

Another approach is just adding `@diff.plus` and `@diff.minus` to the
native themes. Let me know if I should pursue this instead.

Before
<img width="668" height="328" alt="Screenshot 2025-09-18 at 11 16 14 AM"
src="https://github.com/user-attachments/assets/d9a5b3b5-b9ef-4e74-883f-831630fb431e"
/>

After
<img width="1011" height="404" alt="Screenshot 2025-09-18 at 12 11
15 PM"
src="https://github.com/user-attachments/assets/9cf453c0-30df-4d17-99e9-f2297865f12a"
/>
<img width="915" height="448" alt="Screenshot 2025-09-18 at 12 12 14 PM"
src="https://github.com/user-attachments/assets/9e7438a6-9009-4136-b841-1f8e1356bc9b"
/>



Closes zed-industries/extensions#490


Release Notes:
- Fixed highlighting for addition and deletion for diff language

---------

Co-authored-by: MrSubidubi <finn@zed.dev>

Author: 2 people

crates/languages/src/diff/highlights.scm

@@ -3,12 +3,14 @@
 [
   (addition)
   (new_file)
-] @diff.plus
+] @string
+;; TODO: This should eventually be `@diff.plus` with a fallback of `@string`
 
 [
   (deletion)
   (old_file)
-] @diff.minus
+] @keyword
+;; TODO: This should eventually be `@diff.minus` with a fallback of `@keyword`
 
 (commit) @constant
 
@@ -18,8 +20,6 @@
   "diff" @function
   (argument) @variable.parameter)
 
-(filename) @string.special.path
-
 (mode) @number
 
 ([