Novel GraphQL features

GraphQL by PoP strives to provide brand-new features as opt-in, before these are supported by the GraphQL spec.

For instance, nested mutations have been requested for the spec but not yet approved. GraphQL by PoP complies with the spec, using types QueryRoot and MutationRoot to deal with queries and mutations respectively, as exposed in the standard schema.

However, thanks to using the code-first approach, the schema can be automatically transformed, and both queries and mutations will instead be handled by a single type Root, providing support for nested mutations.