ExtendedPromiseInterface: type-hint for then()

[Thomas-Gelf]

This solves the problem that a PHP IDE always believes your then() method returns a PromiseInterface and not an ExtendedPromiseInterface

[SimonFrings]

@Thomas-Gelf thanks for looking into this :+1:

It's not 100% guaranteed that this will always return a ExtendedPromiseInterface. I discussed this with @clue and maybe there should be something like @return PromiseInterface || ExtendedPromiseInterface inside the docblock of the then() method inside PromiseInterface. This should lead to the same results as your addition.

What do you think about this?