Add script function to work with item_group

[lucasgolino]

Pull Request Prelude

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

Changes Proposed

This PR aims to create a script function to get all items without duplicated from a specific group_id. The idea behind this is be able to get all items and be capable to create more complex NPC with it. Ideal for this is: use more groups feature to handle with a list of group in a easiest way without needed to create arrays of a item listing all itens you need, and instead of it, use groups list as resource in NPC.

In same PR i create an utilities tool set to clean duplicated from group in order to get only unique list from group.

Note: I add script command on docs/script_commands.txt

Example:

getitemgroupitems(.@item_list[0], "Old_Card_Album");
for ( .@i = 0; .@i < getarraysize(.@item_list); .@i++ ) {
    mes "^1E56BD\t"+getitemname(.@item_list[.@i])+" ^000000";
}