Add power switch (address 0) (#19)

This is based on the following stanza from my existing custom modbus integration:

```
      switches:
        - name: ftx_power
          slave: 1
          address: 0
          write_type: coil
          scan_interval: 5
          verify:
```

The effect of calling this is like using the "power on/off" button on the Heru remote.

This is the only omission I can find, now that I am trying to move away from my custom integration to this.

custom_components/heru/const.py

@@ -363,6 +363,13 @@
 
 
 HERU_SWITCHES = [
+    {
+        "name": "Power",
+        "modbus_address": "0x00001",
+        "address": 0,
+        "icon": ICON_SWITCH,
+        "register_type": COIL,
+    },
     {
         "name": "Overpressure mode",
         "modbus_address": "0x00002",