PSR12/FileHeader: bug fix - false positives on PHP 8.2+ readonly classes

As reported in #3799 (comment), the `FileHeader` sniff did not take the new PHP 8.2+ `readonly` OO modifier keyword into account.

Fixed now.

Includes test.

src/Standards/PSR12/Sniffs/Files/FileHeaderSniff.php

@@ -188,6 +188,7 @@ public function getHeaderLines(File $phpcsFile, $stackPtr)
 
                 if (isset($commentOpeners[$tokens[$docToken]['code']]) === false
                     && isset(Tokens::$methodPrefixes[$tokens[$docToken]['code']]) === false
+                    && $tokens[$docToken]['code'] !== T_READONLY
                 ) {
                     // Check for an @var annotation.
                     $annotation = false;

src/Standards/PSR12/Tests/Files/FileHeaderUnitTest.18.inc

@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Test;
+
+/**
+ * Sniff should recognize the PHP 8.2 `readonly` keyword as an OO prefix. 
+ */
+readonly class Example
+{
+    public function test() : void
+    {
+        echo '123';
+    }
+}