PHP 8.2 support and bugfixes

• Full support for PHP 8.2.
• New ReaderConfiguration property: $xml_reader_flags (settable via setXmlReaderFlags())
  Will be given to any usage of XMLReader, allowing declaration of libxml constants such as LIBXML_PARSEHUGE.
• Added dependency to ext-libxml in composer.json. (Was already required before, but wasn't explicitly noted as such.)
• Fixed issues with system date/time format specifications in XLSX file.