Does the same as K_PLUGIN_FACTORY_WITH_JSON but saves you typing the class name twice.
If you look at lxr.kde.org you can see our codebase is full of stuff like
K_PLUGIN_FACTORY_WITH_JSON(NinjaBuilderFactory, "kdevninja.json", registerPlugin<NinjaBuilder>(); )
where we repeat NinjaBuilder twice, so add a nice macro so that we only have to write it once