Support interfaces that implement other interfaces

[obi1kenobi]

It seems that graphql-parser currently does not allow interfaces to extend other interfaces. This is a feature that was released in the most recent GraphQL.js major release, v15, and allows some neat functionality: https://dev.to/mikemarcacci/intermediate-interfaces-generic-utility-types-in-graphql-50e8

Direct link to the PR where this was added to the GraphQL spec: graphql/graphql-spec#373
Link to the GraphQL.js PR where this functionality was added: graphql/graphql-js#2084

I'm still learning Rust but I'm open to helping implement this if the maintainers don't mind reading code from a Rust noob 😃

[tailhook]

This looks like a simple feature. Start with tests. Then update data structures, then you will see compiler errors and it should be easy enough to add grammar rules to fix these compiler errors. Grammar is quite non-trivial Rust code, but these rules should have similar examples in other parts of grammar so should be easy to implement.