Not having prefer-const on the JS side makes JS -> TS refactors really
unreadable. This commit just auto-fixes it so we're using
`prefer-const` everywhere.
It turns out not creating a format on-the-fly breaks deserialization.
We now just use the same custom-rule system that tournaments use.
Some hacks are currently necessary (many tests assume that we're
playing in Anything Goes rather than Custom Game) but we'll work them
out in time.
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.
