[WIP] Add an example project to the main Relay repo

[captbaritone]

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While we have a collection of Relay example apps in https://github.com/relayjs/relay-examples, it would be useful to have a single canonical example app which stays in sync with Relay as we develop it, and which can demonstrate each of Relay's features.

This (currently still WIP) PR aims to stub out such an example app which we can expand over time.