Addressed PHP 8.3 deprecation notices

[sc0ttdav3y]

This PR incorporates the work in #48, and adds additional properties to fix the deprecation notices since PHP 8.3:

[PHP Decprecated] Creation of dynamic property Markdownify\ConverterExtra::$tableLookaheadHeader is deprecated in /vendor/pixel418/markdownify/src/ConverterExtra.php:87
[PHP Decprecated] Creation of dynamic property Markdownify\ConverterExtra::$tdSubstitute is deprecated in /vendor/pixel418/markdownify/src/ConverterExtra.php:96
[PHP Decprecated] Creation of dynamic property Markdownify\ConverterExtra::$tableLookaheadBody is deprecated in /vendor/pixel418/markdownify/src/ConverterExtra.php:98

I have not been able to test via the included tests, as PHPUnit is still at v4 and this is no longer compatible with PHP 8.3 (and I don't have the time to upgrade it in this PR), however this is a minimal change so it should not have any new bugs.