languages: Change syntax highlighting for C/C++ preprocessor directives (#48109)

This changes the highlight capture for preprocessor directives from
`@keyword.directive` to `@preproc` in both C and C++.

PR #44043 changed C from `@keyword` to `@keyword.directive` for
consistency with C++, but `@keyword.directive` is still semantically
wrong. Preprocessor directives are not language keywords — they are
instructions to a separate preprocessing phase that runs before
compilation.

Using `@preproc` reflects this distinction and allows themes to style
them independently from actual language keywords like `const`,
`struct`,`if`, etc. This is consistent with how editors like CLion
handle preprocessor directives.

Before:
<img width="710" height="653" alt="before"
src="https://github.com/user-attachments/assets/5c02fc06-bc19-4112-ae53-ad72eb8044e3"
/>

After:
<img width="710" height="653" alt="after"
src="https://github.com/user-attachments/assets/2490e796-7286-4fbb-81b0-387f551cde8f"
/>

Release Notes:

- C/C++: Syntax highlighting for preprocessor directives can now be
tweaked with @preproc capture group.

Co-authored-by: ozacod <ozacod@users.noreply.github.com>

crates/languages/src/cpp/highlights.scm

@@ -186,7 +186,7 @@ type: (primitive_type) @type.builtin
   "#ifndef"
   "#include"
   (preproc_directive)
-] @keyword.directive
+] @preproc
 
 (comment) @comment