[moves] Implement a move generator for pawns

I used Claude to help me figure this out. First time using AI for coding. It was
actually rather helpful!

Calculate BitBoards representing the various kinds of pawn moves when the move
generator is created, and then iterate through them.

En passant still isn't implemented here. This code has not been tested yet either.

moves/src/generators.rs

@@ -0,0 +1,12 @@
+// Eryn Wells <eryn@erynwells.me>
+
+mod pawn;
+
+pub use pawn::PawnMoveGenerator;
+
+use crate::Move;
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct GeneratedMove {
+    pub(crate) ply: Move,
+}