Implements the "Validation" section of the spec.

Validation runs synchronously, returning an array of encountered errors, or an empty array if no errors were encountered and the document is valid.

A list of specific validation rules may be provided. If not provided, the default list of rules defined by the GraphQL specification will be used.

Each validation rules is a function which returns a visitor (see the language/visitor API). Visitor methods are expected to return GraphQLErrors, or Arrays of GraphQLErrors when invalid.

Optionally a custom TypeInfo instance may be provided. If not provided, one will be created from the provided schema.