技能学习：技能书使用的学习确认

-- 错误：一点即消耗，学错不退
function useBook(p, bookId)
    consumeItem(p, bookId)
    learnSkill(p, BOOKS[bookId].skill)
end

local function useBook(p, bookId)
    local info = BOOKS[bookId]
    if myClass(p) ~= info.cls
        or myLevel(p) < info.need then
        showTip(p, "职业或等级不符，无法学习")
        return
    end
    showConfirm(p, "学习 " .. info.name .. "？", function()
        consumeItem(p, bookId)
        learnSkill(p, info.skill)
        showTip(p, "技能学习成功")
    end)
end