WIP: [R] Add lasagna exercise & initial track setup

[jonmcalder]

• [x] Write a Concept Exercise implementation guide
• [x] Implement lasagna concept exercise
• [ ] Add concept docs
• [x] Update config.json

[ErikSchierboom]

This Friday (Jan 29th 2021), we're going to explode this repo back into the individual track repositories and archive this repo.

As such, this PR needs to be merged by Friday 9am UTC, or it will need to be manually reopened against the track repo.