fix: Avoid a deprecated error when the attribute name is numeric and DirectLex is used

[matsuo]

I have created a pull request for #393.
Issue #393 had shown a case where a deprecated error occurs using PHP 8.1 or later.
This PR avoids that error.

[Example]
Contents of "test-393.php"

<?php
require_once './htmlpurifier/library/HTMLPurifier.auto.php';
$config = HTMLPurifier_Config::createDefault();
$purifier = new HTMLPurifier($config);
$config->set('HTML.AllowedElements', ['a']);
$config->set('HTML.AllowedAttributes', ['href']);
$config->set('Core.LexerImpl', 'DirectLex');
$clean_html = $purifier->purify('<a href="https://example.com/" 10="hoge">Test</a>');
echo $clean_html . "\n";

% /opt/homebrew/opt/php@8.0/bin/php -v | head -n 1
PHP 8.0.30 (cli) (built: Jun  6 2024 19:16:32) ( NTS )
% /opt/homebrew/opt/php@8.0/bin/php test-393.php
<a href="https://example.com/">Test</a>

% /opt/homebrew/opt/php@8.1/bin/php -v | head -n 1
PHP 8.1.29 (cli) (built: Jun  5 2024 05:51:57) (NTS)
% /opt/homebrew/opt/php@8.1/bin/php test-393.php
PHP Deprecated:  ctype_lower(): Argument of type int will be interpreted as string in the future in /Users/(snip)/htmlpurifier/library/HTMLPurifier/Token/Tag.php on line 47

Deprecated: ctype_lower(): Argument of type int will be interpreted as string in the future in /Users/(snip)/htmlpurifier/library/HTMLPurifier/Token/Tag.php on line 47
<a href="https://example.com/">Test</a>

Thank you.

[pyres01]

邮件已收到，谢谢！

[github-actions]

🎉 This PR is included in version 4.18.0 🎉

The release is available on GitHub release

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