Export TypedDocumentString

Right now, if we use [`documentMode: 'string'`](https://the-guild.dev/graphql/codegen/plugins/presets/preset-client#documentmode) 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.