add @mock spec (#10)

see graphql/ai-wg#79

- 🚀 **Rendered Spec:**
https://public.larah.me/~mark/MockSpec.html
- ✨ **Demo**: https://mock-spec-demo.larah.me/

Prior art!
https://medium.com/airbnb-engineering/graphql-data-mocking-at-scale-with-llms-and-generatemock-30b380f12bd6

Author: magicmark