Definition mixins

[Christopher Thorn]

JSON is not a good serialization format for chess positions or games.
We have FEN and PGN for those use cases. Implementing the "business
logic" for chess is quite a lot more challenging than implementing FEN
or PGN (de)serialization.

Mixins are well-suited for writing chess software in JavaScript. For
instance, a Knight is a "leaper", Bishops, Rooks and Queens are
"riders". People have imagined many fairy pieces[^1], which mix
various combinations of traits.

[^1]: https://en.wikipedia.org/wiki/Fairy_chess_piece