refactor: rewrite weather.lua to use WeatherAPI

When I looked into OpenWeatherMap 3.0 API they demanded credit card
information even for their free tier. Therefore I decided to move to
another provider with a generous tier.

In a first step I rewrote the current weather report (i.e. no forecast)
to use it.

I haven't received any commentary on #442 so I don't know where your
mind was going. If OpenWeatherMap phases out its 2.5 API and nobody else
intends to use their 3.0 one, I offer to take this implementation
instead.

Signed-off-by: André Jaenisch <[email protected]>