`item.itemUser` replaces `item.zMoveUser`, and is a new field
representing a list of possible users of an item, for items that are
restricted to specific species.
`item.zMoveUser` is a base forme for items that affect every forme, and
a forme name otherwise (forme name is currently only used for
`forcedForme` uses).
Arceus is a weird case. Both generic Z crystals and Plates, while
having unique effects on Arceus, also have effects on other Pokémon.
For this reason, Arceus isn't listed as an `itemUser` of either of
these item types - use-cases that want this should also check
`item.forcedForme`.
This is mostly intended to be useful to move-searching and custom
rules (like NatDex, which will only allow past-gen items associated
with removed species).
`import =` and `export =` are really only intended for backwards
compatibility with CommonJS. While I really don't like the new module
system TC39 has designed for us, it's what we should be using, and
consistency is important.
