Add a helper function to get worldedit positions

[IntegratedQuantum]

Currently this code needs to be in every command that edits the selection:

const pos1 = source.worldEditData.selectionPosition1 orelse {
	return source.sendMessage("#ff0000Position 1 is not set.", .{});
};
const pos2 = source.worldEditData.selectionPosition2 orelse {
	return source.sendMessage("#ff0000Position 2 is not set.", .{});
};

const posStart: Vec3i = @min(pos1, pos2);
const posEnd: Vec3i = @max(pos1, pos2);

extracted from #2797