In most other similar systems, like TeamValidator, we use `thing.dex` instead of having it extend `ModdedDex`. Battle has always extended `ModdedDex`, though. This changes Battle to match the others.
This should fix an issue with `Battle.data` not being cached.
This also frees up Battle to extend ObjectReadWriteStream<string> in a future update.
`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.
