Variant-specific endgame code

[ianfab]

For built-in variants support variant-specific endgame code in order to collect variant endgame knowledge in one place and to improve playing strength. Examples:

• Makruk https://github.com/ianfab/Makruk-Stockfish/blob/da3b89d87c21b9bfc2ea38927e2b22e8b336f926/src/endgame.cpp#L89-L104
• Shatranj https://github.com/ianfab/Shatranj-Stockfish/blob/218ee0a6dec87cf77e09a30947700fc794fd3d5d/src/endgame.cpp#L80-L90
• Atomic https://github.com/ddugovic/Stockfish/blob/8a59ec9844987ca493106465de9029b6ec59b574/src/endgame.cpp#L97-L102
• Antichess https://github.com/ddugovic/Stockfish/blob/8a59ec9844987ca493106465de9029b6ec59b574/src/endgame.cpp#L92-L94
• S-Chess https://github.com/ianfab/Seirawan-Stockfish/blob/dad6e2154ba48f2ff9427fc3923ea2282b74f9e1/src/endgame.cpp#L101-L102
• Misere https://github.com/fairy-stockfish/Fairy-Stockfish/blob/c3228759b00fd801af802288f3b396364346332b/src/endgame.cpp#L147-L178