Add syntax highlighting for named arguments (#95)

## Summary

Adds syntax highlighting for PHP 8+ named arguments in function/method
calls.

Named arguments like `id:`, `firstName:`, `lastName:` in:
```php
$this->users->map(fn (User $user) => new UserDTO(
    id: $user->id,
    firstName: $user->first_name,
    lastName: $user->last_name,
));
```

Are now highlighted with the `@label` token, which themes can style
distinctly (e.g., JetBrains IDEs use a muted blue for these).

## Implementation

Added a simple tree-sitter query that captures the `name` field of
`argument` nodes:

```scheme
(argument
  name: (name) @Label)
```

The tree-sitter-php grammar already exposes the `name` field on
`argument` nodes for named arguments, so no grammar changes are needed.

## Test plan

- [x] Tested locally with `zed: install dev extension`
- [x] Named arguments now highlight correctly in PHP files

Author: alfredomtx

languages/php/highlights.scm

@@ -8,6 +8,11 @@
 (named_type (name) @type) @type
 (named_type (qualified_name) @type) @type
 
+; Named arguments (PHP 8+)
+
+(argument
+  name: (name) @variable.parameter)
+
 ; Functions
 
 (array_creation_expression "array" @function.builtin)