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.
This is a surprisingly minor refactor considering how many files it
touches, but most of this is only renames.
In terms of file renames:
- `tools.js` is now `sim/dex.js`
- `battle-engine.js` is now `sim/index.js` and its three classes are
in `sim/battle.js`, `sim/side.js`, and `sim/pokemon.js`
- `prng.js` is now `sim/prng.js`
In terms of variable renames:
- `Tools` is now `Dex`
- `BattleEngine` is now `Sim`
- `BattleEngine.Battle` is now `Sim.Battle`
- `BattleEngine.BattleSide` is now `Sim.Side`
- `BattleEngine.BattlePokemon` is now `Sim.Pokemon`
