Export TypedDocumentString

[jyasskin]

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Right now, if we use documentMode: 'string' the graphql-codegen tool will generate a definition for a TypedDocumentString type into our project. That makes it hard to name the same type from a library like https://github.com/octokit/graphql.js that wants to pass a string through instead of parsing it to a DocumentNode first. It's not obvious that this is the right home for that type, but it seems like the closest existing library.