Fix: Pawns on edge ranks should have no chess.moves()

[some1else]

Closes #577

Pawn on H8 throws a BigInt error when asked about chess.moves({ square: 'h8' })

TypeError: Cannot mix BigInt and other types, use explicit conversions
    at Chess._movePiece (/app/node_modules/src/chess.ts:1737:24)
    at Chess._makeMove (/app/node_modules/src/chess.ts:1764:10)
    at Chess._moveToSan (/app/node_modules/src/chess.ts:2279:10)
    at new Move (/app/node_modules/src/chess.ts:160:35)
    at /app/node_modules/src/chess.ts:1452:34
    at Array.map (<anonymous>)
    at Chess.moves (/app/node_modules/src/chess.ts:1452:20)

[neofight78]

I'm not sure whether to merge this yet, as a pawn on h8 is not a legal position. It maybe better to tighten up / change the way the board can be set up.

[some1else]

Depends on how strictly you'd like to lock the library down to modern chess rules. There are fringe regimes such as the British Chess Association "dead pawn" rule, and the Burmese Chess promotion rules, under which a pawn can legally occupy the promotion square: https://en.wikipedia.org/wiki/Promotion_%28chess%29

You'll want to reconcile the chosen policy with { skipValidation: true } options for the constructor, as that is likely another code path through which it's possible to place a pawn there.

I'm prototyping a rules-free software chess set mimicking a physical board & pieces, where it's in line with the concept to allow such positions: https://shah-baby.netlify.app. It'd be nice if I could still use chess.js for state management.