Add syntax highlighting for named arguments (PHP 8+)

[alfredomtx]

Summary

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

Named arguments like id:, firstName:, lastName: in:

$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:

(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

• Tested locally with zed: install dev extension
• Named arguments now highlight correctly in PHP files