Improved energy reporting for Zaptec Go 2

[TMBom]

I installed my Zaptec Go 2 recently and noticed that the energy was only reported to Homey once per hour. I wanted to get live energy reporting, so that Homey can calculate the correct price based on 15 minute dynamic pricing. It turned out this was not really an issue with the app itself, but Zaptec only updates the signed meter value once per hour in their API.

This pull request contains the following changes for the Go 2:

• The meter_power is now a calculated value based on the signed meter value + the energy of the current session. While a car is connected, the app will increment this value based on the current session. If there is no car connected and a new value for the signed meter is received, this meter_power is overwritten with the signed meter value to adjust for potential rounding issues that may have occured during the increments in the charing session.
• By using the meter_power Homey now correctly reports the costs of charging and this also enables support for the Power by the Hour Homey app.
• To preserve the total energy this year, this is now a separate cability with the name meter_power.total_this_year. This is still retrieved only once a day.
• The current for each phase is only updated once every 30 seconds. This is sufficient for most scenarios, but when you want to do load balancing with Homey and adjust the current limit, it is relevant to get these values sooner. With this change, after setting the current, the app will retrieve the new values after 7 and 12 seconds in addition to the 30 seconds interval. If you do load balancing every 15 seconds, this means that the charger should have adjusted the current and that in the next iteration you should have up to date values.
• The build command in the package.json is updated. I could not get it to work without changing this, but if it works for others, please just discard this change.

Note that all changes are only for the Go 2. I do not have other Zaptec chargers and most other chargers do not have a signed meter, so I'm not sure if these changes could also work for other chargers. I have tested the code in the pull request for several weeks and it works fine for me.

This is my first pull request for a Homey app, so if I need to change anything, just let me know. Happy to contribute more in the future.