Add a new public API function `wrenReturnBool` to the Wren C API, enabling
foreign method implementations to return boolean values. The implementation
stores the boolean as a Wren BOOL_VAL in the foreign call slot and clears
the slot pointer, following the same pattern as existing `wrenReturnString`.
Also add the missing `prep` target to the .PHONY list in the Makefile and
include `<stdbool.h>` in the public header to support the `bool` type used
in the new function signature.