[core] Declare Rank::PAWN_STARTING_RANKS

This is a slice that declares the pawn starting ranks for each color.

core/src/coordinates.rs

@@ -159,6 +159,15 @@ impl Rank {
         Rank::SEVEN,
         Rank::EIGHT,
     ];
+
+    /// Ranks on which pawns start, by color.
+    ///
+    /// ```
+    /// use chessfriend_core::{Color, Rank};
+    /// assert_eq!(Rank::PAWN_STARTING_RANKS[Color::White as usize], Rank::TWO);
+    /// assert_eq!(Rank::PAWN_STARTING_RANKS[Color::Black as usize], Rank::SEVEN);
+    /// ```
+    pub const PAWN_STARTING_RANKS: [Rank; 2] = [Rank::TWO, Rank::SEVEN];
 }
 
 #[rustfmt::skip]