fix: parse minimal pgns

[theborakompanioni]

This is currently WIP.

Goal of this PR is being able to parse minimal PGNs, e.g. starting positions (without any moves made yet).
Any feedback is highly appreciated.
Q: What is the most minimal valid PGN?
From what it seems, an empty string '' is a valid PGN, as mentioned in Section 8 of the spec, hence it should be supported.

"An empty file is a valid, although somewhat uninformative, PGN database"

The second minimum input would be just a Termination Marker, e.g. simply *.

This is for example how chess.com exports the starting position:

[Event "?"]
[Site "?"]
[Date "????.??.??"]
[Round "?"]
[White "?"]
[Black "?"]
[Result "*"]

*

In all the cases described above, the loadPgn method throws an error:

Invalid move:

      1220 |     if (!moveObj) {
      1221 |       if (typeof move === 'string') {
    > 1222 |         throw new Error(`Invalid move: ${move}`)
           |               ^
      1223 |       } else {
      1224 |         throw new Error(`Invalid move: ${JSON.stringify(move)}`)
      1225 |       }