Function perft

perft(board: BitboardState, depth: number): number
Move-generation node count: 1 at depth 0, else the sum over every legal move. The position is played and taken back as it goes, so it reads identical afterwards. Counts match the published perft numbers, which is the whole point: a wrong move generator cannot hide from them.
Parameters
- board: BitboardState
- depth: number
Returns number
Example
```
import { perft, startingBitboard } from '@datamoc/mwg/board';

console.log(perft(startingBitboard(), 2)); // 400 - the opening's two-ply count
```
- Defined in src/board/bitboard.ts:520