Adds a global variable to the QuickJS context called XSCRIPTCONTEXT which holds an instance of ScriptContext. This makes the scripts more compatible with scripts written for the Rhino JS implementation and also more like Python scripts. Change-Id: I95fd86229786a37c6fff282f45f65555cbde21cf Reviewed-on: https://gerrit.libreoffice.org/c/core/+/205122 Tested-by: Jenkins Reviewed-by: Neil Roberts <nroberts@libreoffice.org>