Export validation functions for FeasiblityStudy

[rschiffer]

Export getYieldClass & getNormalStock in a way it can be reused e.g. for form validation.

          > > It's kinda wild but I can't think of an simpler way

I know... It's basically the validation of the forest-management repo. Would it make sense to export the validations in a function to reuse them here? Should I create an issue for that? Or do we even want that?

I think that that's a great idea, I'd say that we should open an issue for it

Originally posted by @raulsteurer in https://github.com/treely/app/issues/823#issuecomment-1742997439