Add VoxelArea() constructor

[TurkeyMcMac]

VoxelArea objects can now be created with VoxelArea(MinEdge, MaxEdge). This VoxelArea constructor format is more consistent with VoxelManip. It also lets you create an area like so:

local area = VoxelArea(vm:get_emerged_area())

To do

This PR is Ready for Review.

How to test

You can use this command:

minetest.register_chatcommand("voxelarea_constructor", {
	func = function(name, param)
		local player = minetest.get_player_by_name(name)
		local pos = player and player:get_pos()
		if not pos then return false, "No player" end
		local vm = VoxelManip(pos, pos)
		local va = VoxelArea(vm:get_emerged_area())
		return true, tostring(va.MinEdge) .. "\t" .. tostring(va.MaxEdge) .. "\t" .. va:getVolume()
	end,
})

You can also test mods that use the old constructor to make sure they still work.

[SmallJoker]

Seems helpful.