[Rebase] Implement check_offset for decorations

[Montandalar]

This is purely a rebase of #13152 - all credit goes the original author @nephele-gh.

This rebase was discussed on the Minetest Matrix before I did it and the author gave permission. I don't meant to step on anyone's toes, I just wanted to keep an otherwise perfectly fine PR moving. If this is not the way to do things in future, that's fine. On the flipside, if there are other PRs that would like this, I could help with those.

[SmallJoker]

Works. Test with the following code:

minetest.register_decoration({
	name = "DEMO CHECK OFFSET",
	deco_type = "simple",
	place_on = {"default:sand"},
	spawn_by = {"default:sand"},
	fill_ratio = 10,
	decoration = "default:wood",
	check_offset = 1,
	height = 10,
	num_spawn_by = 5,
})

This requires results in some sort of steep hill detection which might be interesting for plant spawning. Here's the result of altering the check_offset value: