Recent versions of `npm` record sub-dependency version ranges
differently, though the actual modules installed have not changed. I'm
separating this into its own commit so it doesn't make other changes
harder to see.
Match the versions of Webpack, Babel, and related plugins as used in
scratch-gui. That plus a few other tweaks means that this repository
builds the same with or without `npm ln scratch-gui`, and also means
that all syntax used in scratch-gui should build the same here.
The old version of webpack takes a long time and then runs out of memory
when I add media library assets in `src/static`. The new version handles
it OK.
