The previous configuration mixed Webpack output with static content in
order to create the playground. This change moves that static content
from `/playground/` into `/src/playground/` and adds a Webpack rule to
copy it into the playground as part of the build.
On the surface this might seem unnecessary, but it provides at least two
benefits:
- It's no longer possible to accidentally load stale build output
through `webpack-dev-server` in the event of a misconfiguration. This
was very easy in the previous configuration, and might in fact be the
only way that `webpack-dev-server` ever worked for this repository.
- It's simpler to ensure that various rules apply to the hand-authored
content and not build outputs. This includes lint rules, `.gitignore`,
IDE symbol search paths, etc.
