Support custom pawn pieces

Support custom pawn pieces

This introduces support for
* initial moves, promotions, and en passant for all pieces.
* arbitrarily shaped double/triple step and promotion regions.
* per color configuration of promotion types and zone, etc.

These new configuration options enable support for
* Berolina, Spartan, Wolf
* Torpedo, pawnsideways, pawnback
* Legan, Troitzky

Most changed configuration options are kept as deprected options for now
for backwards compatibility, but it is recommended to migrate variant
configuration files as the deprecated options might be removed later.

Closes #305, closes #4, closes #209, closes #260, closes #51, closes #328, closes #596.

No functional change for other variants.