If an extension worker does not complete initialization in a reasonable
amount of time (currently 15 seconds) then the worker will be
terminated. A complete initialization includes registering an extension.
WeDo2 and Pen blocks have been converted to internal extensions, and can
now be loaded by giving `loadExtensionURL` the string 'pen' or 'wedo2'
instead of an actual URL.
The new `_registerInternalExtension` method on the extension manager
will register an extension object (an object with a `getInfo()` method)
with the extension system without sandboxing the object in a Worker.
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.
