The playground was trying to build with a now-missing entry point.
Also, I fixed some JSDoc comments, added HTML labels, etc. until IDEA
had no meaningful complaints about `playground/index.html`.
The playground now must be loaded with a URL that ends in "/playground/"
or "/playground/index.html"; loading at "/" will no longer work.
The /playground folder is now purely an output of the build process,
rather than being a mix of build output and static content. The static
content that was previously there (namely, index.html) has been moved
into /src/playground. The `contentBase` has been set to `false` to avoid
confusion between current, in-memory build output and stale, on-disk
build output.
