Add item action/condition types related to item cooldowns

[eggohito]

This PR adds an item action type, and two item condition types, related to modifying and checking the (absolute or relative) cooldown of an item stack respectively:

---

The modify_item_cooldown item action type modifies the cooldown duration of the item stack. It accepts these fields:

Field	Type	Default	Description
modifier	Modifier	optional	If specified, this modifier will be applied to the cooldown duration of the item stack.
modifiers	Array of Modifiers	optional	If specified, these modifiers will be applied to the cooldown duration of the item stack.

---

The item_cooldown item condition type compares the cooldown duration of the item stack. It accepts these fields:

Field	Type	Default	Description
comparison	Comparison		Determines how the cooldown duration of the item stack should be compared to the specified value.
compare_to	Integer		The value at which the cooldown duration of the item stack will be compared to.

---

The relative_item_cooldown item condition type compares the cooldown duration percentage of the item stack (a value ranging from 0.0 (0%) to 1.0 (100%).) It accepts these fields:

Field	Type	Default	Description
comparison	Comparison		Determines how the cooldown duration percentage of the item stack should be compared to the specified value.
compare_to	Float		The value at which the cooldown duration percentage of the item stack will be compared to.