modules/system: Add System.exit() API
modules/system: Add System.exit() API

Kind of lame we didn't have this before, since it's a lot harder to
write "scripts" that start and exit on error without it.

https://bugzilla.gnome.org/show_bug.cgi?id=691447

endlessm/gjs,endlessm/gjs,pixunil/cjs,mtwebster/cjs,endlessm/gjs,djdeath/gjs,djdeath/gjs,pixunil/cjs,djdeath/gjs,pixunil/cjs,endlessm/gjs,mtwebster/cjs,pixunil/cjs,pixunil/cjs,djdeath/gjs,endlessm/gjs,mtwebster/cjs,djdeath/gjs,mtwebster/cjs,mtwebster/cjs