Readonly properties, no __typename option

[calebgregory]

We use Readonly properties and strip out __typename for all Typescript types we generate from GraphQL Schema (using options immutableTypes: true and skipTypename: true). These test factories are a Godsend, and I absolutely love that this library exists. Is it possible to add support for Readonly properties and skipTypename?

[stephenh]

@calebgregory ...um...like a year later, we realized we weren't watching notifications on this repository :-( sorry about that!

But yeah, these are a good idea to implement; happy to help chat through a PR if this is still something at all relevant to you these days :-D