Block string support [just landed in `graphql-js`](https://github.com/graphql/graphql-js/pull/926). Adding this issue so we keep track of progress on adding it to `graphql-ruby`. Reference implementation: https://github.com/graphql/graphql-js/pull/926 Spec: https://github.com/facebook/graphql/pull/327