Add an auto discovery plugin for mixins

[nea89o]

This will automatically find mixins in the mixin plugin, not requiring the manual configuration. This is less friction, especially for newer users. Usually this isn't super advisable, or needs more configuration to avoid server/client mixins being incorrectly applied, but with the target demographic of mostly client mods, I think this is a worthwhile change.

[nea89o]

I'm gonna tag @ILikePlayingGames since they have some experience with both this template and forge modding in general, but other people feel free to chip in what you think. Should this be default installed in this template or should this be in an external tutorial.

[hannibal002]

LGTM