Readonly variables have "Missing member variable doc comment"

Problem/Motivation

readonly class member variables generate `Missing member variable doc comment` errors, regardless of whether they actually have a doc comment.

Steps to reproduce

Here's code that causes the sniff error:

<?php

namespace MyNamespace;

/**
 * Test class with a test member variable.
 */
class Test {

  /**
   * Test.
   *
   * @var float
   */
  public readonly float $test;

}

Proposed resolution

readonly member variables with doc comments shouldn't show errors like that.

This can be fixed by adding T_READONLY to the ignore list in processMemberVar() in VariableCommentSniff.php