`extends` error when using a `type` alias

[CZDanol]

Bug description

Seems like the extends syntax doesn't accept interface type aliases.

Steps to reproduce

interface IDFOutputT<T> {}
type IDFNumberOutput = IDFOutputT<DFNumberType>;
class DFNumberValue implements IDFNumberOutput {}

ERROR TS2422: A class can only implement an interface.

 class DFNumberValue implements IDFNumberOutput {
                                ~~~~~~~~~~~~~~~

AssemblyScript version

v0.24.1