Tracking the version under one `-prerelease` channel allows other packages to depend on `^0.1.0-prerelease` for the most up to date version.
Track the git sha for the release in package.json
Reuse the json package needed for writing the sha to retrieve the package.json version field
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.
