Add action modbus.write_registers

There are modbus servers, that only support write_registers(), not write_register(), namely Nibe Modbus40. Therefore, it is impossible to set any modbus values with action. This should be easy implementation, since this already exists in the modbus integration. In Switch you can define the write_type to ‘holdings’ instead of ‘holding’ and in that case it will use write_registers() instead of write_register()