Add nullability directives

[martinbonnin]

Introduce 2 directives used to help working with nullability.

@semanticNonNull

type User {
    # generate email as non-null String in Kotlin even though its type is nullable
    email: String @semanticNonNull
}

@semanticNonNull is the third state found in proposals such as SemanticNonNull except it adds no new GraphQL syntax.

@semanticNonNull can be used in the server schema but also as client extensions:

extend type User @semanticNonNull(field: "email")

@catch

{
  # generate user as `FieldResult<String, Error>` in Kotlin to have access to the error if needed
  user @catch(to: RESULT) {
    name
  }
}

@catch can also be used to fail the whole response in case there is an error in one field:

{
  # fail the response if user or name has an associated error
  user @catch(to: THROW) {
    name
  }
}

[BoD]

✅

[hwillson]

Looks great @martinbonnin - thanks for working on this!