FmlWriter produces incorrect output for bare ModifierChange.REMOVE

[Juuxel]

When you have AccessTransform.of(ModifierChange.REMOVE) in an access transform set and write that set using FmlWriter, the resulting AT file contains a broken line starting with -f.

This is not supported by the AT parser and lexer used by FML, which always expects an explicit access modifier as well.

I'm not sure if there's any good way to fix this, though, since it should probably be using the existing visibility of code in question, but that info isn't available here. In Architectury Loom, we just made it default to public-f, but that might also be unwanted.