Getter on fields for introspection types

[lennyburdette]

Description

When traversing through selection sets, it's handy to know if a field is part of introspection so I can ignore it:

match selection {
    Selection::Field(f) => {
        if !f.is_instrospection() {
            // do something with a field that has a resolver           
        }

GraphQL-js has isIntrospectionType so a getter on output types could be useful too.