[board] Implement Default for Position

Default returns an empty position.

board/src/position/position.rs

@@ -193,6 +193,12 @@ impl Position {
     }
 }
 
+impl Default for Position {
+    fn default() -> Self {
+        Self::empty()
+    }
+}
+
 impl fmt::Debug for Position {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut output = String::new();