Generalize code for game resources

[SamTheBlow]

Examples of game resources: money, iron, happiness, action points, RISK cards...
These all work mostly the same way, so they should all be represented by a single class.

There are already some traces of this generalization in the code, for example with this class:

grand-strategy-game/project/scripts/game_resources/resource_cost.gd

Lines 1 to 18 in 163519f

	class_name ResourceCost
	## Class responsible for calculating how much of given game resource
	## is required for given amount of something.
	var _cost_for_one: float
	func _init(cost_for_one: float) -> void:
	_cost_for_one = cost_for_one
	func cost_fori(amount: int) -> int:
	return ceili(_cost_for_one * amount)
	func cost_forf(amount: float) -> float:
	return _cost_for_one * amount

Once this is implemented, then we can add #150, #152 and much more