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 👍

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?

[SimonFrings]

This pull request here shouldn't be needed anymore with Promise v3 out, as we added template types and improved type safety in general in #246, #247 and others. The functionality of the ExtendedPromiseInterface is now part of PromiseInterface and has been removed in #75.

This is why I'll go ahead and close this ticket here, thanks for the contribution 👍