Determine if an action can be used (you have sufficient mana, reagents and the action is not on cooldown).
isUsable, notEnoughMana = IsUsableAction(slot)
- Number - Action slot to query
- boolean - true if the action is currently usable (does not check cooldown or range), false otherwise.
- boolean - true if the action is unusable beacuse the player does not have enough mana, rage, etc.; false otherwise.