pgn parser accepts ambiguous Nec3

[robertnurnberg]

As discussed on discord the pgn parser of chess-library accepts this ambiguous SAN move when reading the game from #144.

It only fails later, when it cannot parse Nd4+, because internally it moved the wrong knight.

Ideally the parser should throw the error (and stop parsing) at the first ambiguous move, as do python-chess and lichess analysis board, for example.

[Disservin]

not a pgn parser issue but a san parser issue