LibChess: Include pawns in FEN output

Previously, the initial position would look like this:

rnbqkbnr//8/8/8/8//RNBQKBNR w KQkq - 0 1

Now, we correctly give pawns the P/p character in FEN output:

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

Also, we only ever have 1 or 0 characters for a piece, so let's return
`Optional<char>` instead of `StringView` from `char_for_piece()`.

Userland/Libraries/LibChess/Chess.h

@@ -25,7 +25,11 @@ enum class Type : u8 {
     None,
 };
 
-StringView char_for_piece(Type type);
+enum class Notation {
+    Algebraic,
+    FEN,
+};
+Optional<char> char_for_piece(Type, Notation);
 Chess::Type piece_for_char_promotion(StringView str);
 
 enum class Color : u8 {