Add move generation for castling

src/game.rs

@@ -70,6 +70,10 @@ impl State {
     pub const fn get_en_passant_target_square(&self) -> Option<u8> {
         self.en_passant_target_square
     }
+
+    pub const fn get_castling_ability(&self) -> u8 {
+        self.castling_ability
+    }
 }
 
 use std::fmt;
@@ -87,3 +91,10 @@ impl Default for State {
         Self::new()
     }
 }
+
+pub enum CastlingRights {
+    Short,
+    Long,
+    Both,
+    None,
+}