If another package wants to import `src/index.js`, it needs to replicate the loader config in `webpack.config.js`. To avoid this, package the module in `dist.js` and set that as the module to require.
I changed `main` to point to this so that other packages can just `require('scratch-blocks')`, but it would also work for our purposes to add `"webpack": "./dist.js"` if we want to keep `src/index.js` as `main`.
The build generates a new output set called `render-webgl-worker`, meant
to be imported from a web worker. This exposes the `RenderWebGLRemote`
class to facilitate communication with the renderer using `postMessage`
and `onmessage`.
Only a few messages are implemented so far, but it's enough to run the
demo page.
