Use .afcy for type-annotated FlatCurry files

The most recent version of flatcurry-annotated expects annotated FlatCurry files to have the extension .afcy:

https://git.ps.informatik.uni-kiel.de/curry-packages/flatcurry-annotated/-/blob/2df4dbd6d5b89d9da4a1bb20b4d25cd53d4776ff/src/FlatCurry/Annotated/Files.curry#L23

This branch therefore applies the same change in the frontend (.tafcy to .afcy).