- Remove WeDo 2 extension from the runtime's default block packages list
- The WeDo 2.0 extension now calls its own `connect` method on startup
I also renamed `EXTENSION_NAME` to `EXTENSION_ID` for consistency with
the rest of the extension system.
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.
