Change IMappingFile.getField to require descriptor

[LexManos]

This isn't important in Minecraft world as Proguard doesn't reuse names for fields in the same class.
However for completeness sake we should do this.
It would be nullable, and obviously fallback to the insensitive map if the loaded mappings do not contain field types.

This is most likely {to prevent having to do hacky stuff} a breaking change, as we'd remove the non-descriptor function.