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.
The loom mixin extension adds the mixin to the launch arguments automatically. Adding it in the launch config causes the argument to be added twice, which breaks launch logic. Adding the argument twice causes the first command line argument after the second mixin addition to be parsed incorrectly and therefore ignored.
