Fix parsing of GraphQL Interfaces

[execveat]

Write test cases and implement support for interfaces. The basic functionality should work already, but some fields are only reachable by using ... on syntax.

Documentation on interfaces:

• https://www.apollographql.com/docs/apollo-server/schema/unions-interfaces/#querying-an-interface
• https://graphql.org/learn/schema/#interfaces
• https://dgraph.io/blog/post/unions-interfaces/

[execveat]

GQLSpection has implemented support for generating queries from a schema with interfaces. However, right now interfaces get parsed incorrectly in the editor tab:

query GetSearchResults {
  search(contains: "Shakespeare") {
    __typename
    ...
    on
    Book {
      title
    }
    ...
    on
    Author {
      name
    }
  }
}

[execveat]

I completely rewrote the request parser, it should fix this.