RichString::setHtml doesn't support <sup> and <sub> tags

[call-me-xequence]

The code below doesn't work properly: sup and sub tags are ignored

  QXlsx::Document xlsx;
  QXlsx::RichString rich;
  rich.setHtml( "m<sup>2</sup>")
  xlsx.write("A1", rich);
  xlsx.saveAs("richtext1.xlsx");

[dantti]

please run clang-format