added isKey itemtype

[ghost]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Issues addressed:
now you can add numbers to a key using /i silver key, 1234 for example.

[Zbizu]

tbh I expected this game to have more keys, but either way it delivered

new /i works fine (tested with stackable items, unstackable items, fluid containers and keys)

[Zbizu]

source edit is redundant as you can simply define function ItemType:isKey() return self:getType() == ITEM_TYPE_KEY end

[DSpeichert]

drop the C++ change in favor of Lua as zbizu pointed out