feat(php): add highlight tests

tests/query/highlights/php/variables.php

@@ -0,0 +1,28 @@
+<?php
+
+class A {
+  public function foo(self $a): self {
+//                          ^ @variable
+    new self();
+//      ^^^^ @constructor
+    new static();
+//      ^^^^^^ @constructor
+    new parent();
+//      ^^^^^^ @constructor
+    $this->foo();
+//   ^^^^ @variable.builtin
+//       ^^ @operator
+//         ^^^ @function.method.call
+    self::foo();
+//  ^^^^ @variable.builtin
+//        ^^^ @function.call
+    static::foo();
+//  ^^^^^^ @variable.builtin
+    parent::foo();
+//  ^^^^^^ @variable.builtin
+    $this->foo;
+//         ^^^ @variable.member
+    $this->foo(a: 5);
+//             ^ @variable.parameter
+  }
+}