Add an easy toggle for mixin/mixinextras in vanilla mode

[Tslat]

Vanilla mode being the obvious use-case for ML projects in the common branch - it makes sense that MDG doesn't supply mixin/mixinextras in that mode due to it being intended for use as a no-frills deployment

However with it being the use-case of choice for ML projects, it is pretty annoying having to manually add mixin/mixinextras when in vanilla mode, given that both fabric and neoforge have it already in their respective modules

Is it possible to add some kind of easy toggle/method call to tell MDG in vanilla mode to include mixins?