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.
Also move mods/ to data/mods/
This makes PS more monorepo-like. The intent is to further separate
the sim and the server code, but without fully committing to splitting
the repository itself.
We now support `./pokemon-showdown start` in addition to
`./pokemon-showdown`. I'm not clear which I want to be the default
yet.
