Improve C and C++ syntax highlighting (#25325)

Release Notes:

  - Improved C and C++ syntax highlighting.

| Zed 0.174.6 | With this PR |
| --- | --- |
|
![Image](https://github.com/user-attachments/assets/fa5bf2e0-c682-4e6a-ab64-bb873579d47c)
|
![Image](https://github.com/user-attachments/assets/dc980de9-cd6a-4b0b-8ddc-ac705f687910)
|

- `NULL`, `nullptr`: `constant` -> `constant.builtin`

```cpp
#include <stdbool.h>
int a[] = {true, false};
const int * IDENTIFIER = nullptr;
```

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/languages/src/c/highlights.scm

@@ -104,7 +104,7 @@
   (false)
 ] @boolean
 
-(null) @constant
+(null) @constant.builtin
 
 (identifier) @variable
 

crates/languages/src/cpp/highlights.scm

@@ -158,7 +158,7 @@ type :(primitive_type) @type.primitive
 [
   (null)
   ("nullptr")
-] @constant
+] @constant.builtin
 
 (number_literal) @number