This commit adds a new hub type: PybricksHub. To use the new hub,
you need to have Pybricks firmware installed on your hub, see
https://pybricks.com for further instructions.

For now, the hub is only available in the Node.js, not in the Web-
Bluetooth implementation. Properties like firmware version are not
implemented yet. You can connect, compile, upload and run Python code
and send and recieve data via NUS with one connected Pybricks hub
at a time.

Changes to bluetooth communication according to
https://github.com/pybricks/technical-info/blob/master/pybricks-ble-profile.md#profile-v120

Currently, compatible firmware can be installed from
https://beta.pybricks.com/

The characteristics are part of generic specifications such as
https://btprodspecificationrefs.blob.core.windows.net/assigned-values/16-bit%20UUID%20Numbers%20Document.pdf
Some are also supported by pybricks