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.
