When asked to load an extension, the Extension Manager starts up a new
Worker. That worker runs the message dispatch system as well as an
instance of the new `ExtensionWorker` class, which will load the desired
extension and register it with the extension system.
Extensions, placed in `./src/extensions/*.js`, are now processed by
Webpack as separate entry points and packed into an `extensions`
subdirectory in the output.
Still to do: query an extension's information, including the blocks it
provides, and register that information with the VM, GUI, etc.
