Improve JSON syntax highlighting (#25329)

Release Notes:

  - Improved JSON syntax highlighting.

| Zed 0.174.6 | With this PR |
| --- | --- |
|
![Image](https://github.com/user-attachments/assets/46c8ae89-aca1-4756-b66c-78ccd8f3778d)
|
![Image](https://github.com/user-attachments/assets/3ba5e5db-1467-40d7-a502-2790feec8ad3)
|

- `null`: `constant` -> `constant.builtin`
- `,`, `:`: `punctuation.delimiter`

```json
{
  "property": null,
  "boolean": true
}
```

crates/languages/src/json/highlights.scm

@@ -13,9 +13,12 @@
   (false)
 ] @boolean
 
+(null) @constant.builtin
+
 [
-  (null)
-] @constant
+  ","
+  ":"
+] @punctuation.delimiter
 
 [
   "{"

crates/languages/src/jsonc/highlights.scm

@@ -13,9 +13,12 @@
   (false)
 ] @boolean
 
+(null) @constant.builtin
+
 [
-  (null)
-] @constant
+  ","
+  ":"
+] @punctuation.delimiter
 
 [
   "{"