deps: update graphql to v15 + interfaces can implement interfaces

[n1ru4l]

https://github.com/graphql/graphql-js/releases/tag/v15.0.0

• moves graphql to peerDependencies, which makes it easier to use a newer version (15.x.x)
• add support for specifying interfaces on interface types (see https://github.com/graphql/graphql-js/pull/2084)

I also created my own private release with these changes for testing whether any stuff breaks (https://www.npmjs.com/package/@n1ru4l/gqtx). I did not notice any issues yet.

In addition, I would really appreciate a new release after merging this. 😊

closes https://github.com/sikanhe/gqtx/issues/19

[sikanhe]

@n1ru4l Thanks, will do a release today

[sikanhe]

@n1ru4l done