Add API to weather mod

[sfan5]

resolves #2913, replaces #3020

Key points

• no is_enabled/set_enabled because that's problematic with multiple mods
• API always works, even on singlenode worlds or with enable_weather = false
  • in the latter case it does not do anything and won't interfere (by default)
• only one way to modify behavior: by overriding

How to test

local old_get = weather.get
weather.get = function(player)
	local data = old_get(player)
	data.clouds = data.clouds or {}
	data.clouds.thickness = 0.1
	return data
end

everything is as before except that clouds are paper-thin

[appgurueu]

Tested, works. Code looks fine besides my one minor comment. I like the simplicity of this. The docs are in line with the rest of game_api.txt, but formatted a bit weirdly if you view them as Markdown (for example the spacing used for delimiting will just be collapsed into a single space; the weather.get = function(player) "inline code" should probably be a subheading). Why not strive for proper Markdown formatting here?

[sfan5]

Why not strive for proper Markdown formatting here?

I'll try to reformat it.