Find the closest node accepted by cond under max_cost

Property definitions

a_star $ Node :: find_closest
	# Find the closest node accepted by `cond` under `max_cost`
	fun find_closest(max_cost: Int, context: PathContext, cond: nullable TargetCondition[N]): nullable N
	do
		var path = path_to_alts(null, max_cost, context, cond)
		if path == null then return null
		return path.nodes.last
	end
lib/a_star/a_star.nit:191,2--197,4