[position] Export Position::active_color()

Passes through to the Board method.

position/src/position.rs

@@ -53,6 +53,11 @@ impl Position {
     pub fn board(&self) -> &Board {
         &self.board
     }
+
+    #[must_use]
+    pub fn active_color(&self) -> Color {
+        self.board.active_color()
+    }
 }
 
 impl Position {