[Bug] Prime Hub stays on after software power-off if USB connected.

[laurensvalk]

Describe the bug
When triggering a soft power-off while USB is connected, it does not shut down in the same way as holding the button.

The animation freezes and Pybricks Code stays connected (but fails to run anything).

To reproduce

from pybricks.hubs import ThisHub
from pybricks.tools import wait

hub = ThisHub()

print(hub.system.reset_reason())
wait(100)
hub.system.reset(0) # Already applied support#379, so here doing power-off

# The run animation stays on and freezes

Expected behavior
Clean shutdown, same as holding the button.

Screenshots

[dlech]

0 is supposed to reset the hub, not power off. See #382 for power off.

[laurensvalk]

See #379 as indicated in comment.

[dlech]

We need to add a new method for proper shutdown that calls pbsys_status_set(PBIO_PYBRICKS_STATUS_SHUTDOWN);

[laurensvalk]

This can be closed because reset will not be used. See #379.

See https://github.com/pybricks/pybricks-micropython/issues/58 for graceful shutdown.