Add ParserOutput::addJsConfigVars

The OutputPage has variables for modules, moduleScripts, moduleStyles,
moduleMessages and the config vars, but the ParserOutput is missing the
last one.

With ParserOutput::addJsConfigVars it is possible to add scripts and it
config vars at one place and have not use the MakeGlobalVariablesScript
hook or other ways to get the needed javascript variable in the output.

Change-Id: I6ad61cca76805f6b9d76e053c98c7509c323d5da
Merge "Add ParserOutput::addJsConfigVars"