Add new weight device class (#79185)

homeassistant/components/recorder/statistics.py

homeassistant/components/recorder/websocket_api.py

homeassistant/components/sensor/__init__.py

homeassistant/components/sensor/device_condition.py

homeassistant/components/sensor/device_trigger.py

homeassistant/components/sensor/recorder.py

homeassistant/components/sensor/strings.json

homeassistant/components/sensor/translations/en.json

homeassistant/util/unit_conversion.py

tests/components/sensor/test_init.py

tests/components/sensor/test_recorder.py

tests/util/test_unit_conversion.py