[website] ☂️ Interactive playground for website/docs

We'd like to have interactive playground examples on the relay website and docs. I'm creating this umbrella issue to experiment with planning and iterating in OSS.

Tasks:
- [ ] Build [Sandpack](https://github.com/codesandbox/sandpack) integration for running Relay Compiler & Transforms (there's some progress in https://github.com/voideanvalue/relay/commit/ae2c665cc47b60012e36f14409ac852dd0296f6d)
- [ ] codemirror extension that processes file source and emits generated artifacts
- [ ] codesandbox glue code for taking the generated artifacts and exposing them as the files available for the bundle
- [ ] codesandbox ui changes for showing the generated artifacts
- [ ] [rust -> wasm] make react-compiler-playground package update automatically
- [ ] [rust -> wasm] expose more methods via relay-playground-compiler
- [ ] Decide what GraphQL Server & Schema to use for interactive examples