port from deprecated kjs to qjsengine
for kf6 todo https://phabricator.kde.org/T11604 kjs is deprecated and we want to move away from it. this ports to qjsengine which does what we need for krename just fine and is also supported in qt6 still all use cases of the js plugin still seem to work fine. variable definition, function definition and using either in the replacement. same for the implicit global variables.