Add new attribute ItemTypeAttrPluralName and get attribute plural in item type parser

[EgzoT]

Add new attribute ItemTypeAttrPluralName with get/set methods. Get attribute plural in item type parser. Add possibility to find item type/types by plural name from item types. Add new added functionality to lua functions. Add missing ItemType functionalities to lua functions.

[EgzoT]

It's feature what I want to use in my loot stats module, this functionality prevent the need to use in module additional parser for get "plural" items name, shortening time to parse all needed data from items.xml.

Usage 1 Usage 2