In certain cases, a spread move that KOed an opponent would send an
incorrect message which could crash the client. This fixes that while
also simplifying the code.
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.