Items defining their own usefulness and isNonstandard per mod

[EnPassant4264]

Copy/pasting from the Suggestions thread:

This would add a rating variable to items to be used in making the Popular/regular/specific/usually useless/useless categories, similar to what Abilities have for... I think randbats? Currently, build-indexes is hardcoded for every non-regular item and even runs checks based on generation, which could be moved to the gens' mod folders with this change. The new code would be as easy as a switch statement based on rating with maybe some fall-throughs for Pokemon-specific items.

Additionally, isNonstandard could be set or removed for an item in ModdedItemsData, which requires editing build-indexes and battle-dex-search to create/load the override. Currently, only item descriptions can be changed by mods.

The purpose of these two changes would be allowing mods to create their own item lists in the teambuilder. Mix and Mega, for instance, would appreciate having all the Mega Stones show up at the top rather than only by searching their name and still showing up as Illegal.

I wanted to get approval before implementing it in case it would be rejected for something I don't know well enough to consider, like adding unwanted networking load on battles; that was what happened with my last request. (There will be more of these in the future)