implement function that finds surface blocks out of all blocks inside polygon

[Metriximor]

this should go in polygonal area functionality, surface is defined as the first block that is walkable, bukkit disponibilises a api to do this in world.getHighestBlockAt(x, z)

[Metriximor]

Folks at the papermc discord told me that I should check the chunks are loaded first maybe using async? first implementation shouldn't worry about this though