[feature] Support for iXML #22
Description
Add support for iXML - need to decide on a function signature to call iXML. BaseX provides fn:invisible-xml see: https://docs.basex.org/main/Invisible_XML which has been suggested for XPath and XQuery Functions 4.0, see: https://qt4cg.org/specifications/xpath-functions-40/Overview.html#ixml-functions