This introduces a new class, BattleActions, available as `battle.actions`,
moving all functions from `data/scripts.ts` to `sim/battle-actions.ts`.
This makes it so that "go to definition" will now work correctly for
functions previously in scripts; we no longer need UnimplementedError,
and there's now a clean conceptual separation between `battle` and
`battle-actions` (whereas the previous distinction between `battle` and
`scripts` was basically nonexistent).
This will be a difficult migration if you maintain a fork with custom
scripted mods. I'm sorry! Migration instructions are here:
https://github.com/smogon/pokemon-showdown/pull/8138
