Support for the Strict Nullability draft specification

[kawazoe]

The GraphQL consortium recently announced a new draft to support smarter clients when it comes to error handling. The Strict Nullability specification enables clients to distinguish actual null values in responses from null values caused by server side errors.

Currently, many clients like Relay supports a temporary implementation of this feature with the @semanticNonNull directive. While this isn't the final version, and it appears like the final version might move to a different approach with a server side @ extendedNullability directive, it would be useful to support the draft right now to enable to play with the draft version of this specification.

See #735 for other Relay-specific directives related to those changes.