Automatically insert refmap path into mixins.json if it's not there already #280
Description
On Legacy MDG refmaps are generated but it's up to the user to make sure the path is present in the mixins.json. If MDG were to take care of this there would be less chance of user error and it would reduce manual work (e.g. when re-using project files or using a template). Fabric Loom already does this: https://github.com/FabricMC/fabric-loom/blob/c7accc60a49b086655305597d08b4df87317dce6/src/main/java/net/fabricmc/loom/task/service/MixinRefmapService.java#L124