implement undo method to revert last move in game

[amjadjibon]

Issue #142

• Revert Last Move: Reverts the most recent move in the game.
• Restore Previous State: Updates the position history to reflect the game state before the last move.
• Return Undone Move: Returns the last move that was undone.
• Error Handling: Provides an error if there are no moves to undo.

[silkrow]

Consider the following test case, it fails since the method and outcome of the game are not changed by the updatePosition function properly. I've added some changes to settle this issue. Please check this PR.

        g = NewGame()
	moves = []string{
		"f4", "e6", "g4", "Qh4",
	}

	for _, m := range moves {
		if err = g.MoveStr(m); err != nil {
			t.Fatal(err)
		}
	}

	lastMove, err = g.Undo()
	if err != nil {
		t.Fatal(err)
	}

	if err = g.MoveStr("h6"); err != nil {
		t.Fatal(err)
	}
	if g.Outcome() != NoOutcome {
		t.Fatal("unexpected game outcome")
	}