Add implicitly-returns-nil annotations

[soutaro]

The annotation tells the Array#[] and Hash#[] may return nil.

RBS itself doesn't do anything for this annotation. The type checkers may give precise types of the methods with the annotations.

[hjwylde]

Hey @soutaro, just had a quick question if you don't mind; which cases are you intending this annotation to be used for? We have a user asking whether Array#first should also be annotated with {implicitly-returns-nil}. I've answered that I suspect it is an intentional design choice that it's only added to accesses ([]), but I'd just like to double check with you to find out for certain.

[soutaro]

@hjwylde Hi! Adding the annotation to Array#first and likes would make sense. I had planned to implement support for the annotation in Steep to test the type checking experience with it, but not finished yet...

[ParadoxV5]

I instead prefer something more explicit… a way for fixed-size tuples and records to narrow the return of their #[] and co.… and possibly expand to variable-size Arrays and Hashes.

[hjwylde]

Thanks @soutaro, I'll make a pull request to add a few more for the user then :).

I think I do half agree with @ParadoxV5's that there are use-cases for more elaborate contracts (and I've had some ideas on others for RubyMine), but I think implicitly-returns-nil is good to start and usable right now.