Fixes: #5481
Upgrades `standard` to major version 12
Fixes #5363 I noticed this happening when making some changes on the nextjs.org/learn app. Basically we didn't apply updates when a warning was emitted from webpack. This would cause issues for users using eslint-loader or similar too.
- Implements Lerna - Moves all source code into `packages/next` - Keeps integration tests in the root directory
