Export operation definitions

When using persisted queries but working in a FE-only repo, we end up without the query text available for our mock graphql setup.

I saw [this recent commit: Add a subcommand to dump definitions to file](https://github.com/facebook/relay/commit/1ae14fe737013ac41a105a23a84ce187930fab15).

We're [doing a very similar thing in our fork](https://github.com/atlassian-forks/relay/commit/322a3fb05b478b041651a56d54a1337ed60be669)

This is a gentle request to externalise that subcommand.

We used the query id, but it would also be fine to use the name since there's no risk of versioning conflation in this situation.