Parse uncommon nag symbols (e.g. ±, ∓, ∞...)

[tneisinger]

Add the ability to parse special NAG symbols from this table.

Example:

const pgn = '1. f3 e6 2. g4?? ∓ $5 ∞ Qh4# 0-1';
const [result] = pgnParser.parse(pgn);

result = {
    comments_above_header: null,
    headers: null,
    comments: null,
    moves: [
        { move_number: 1, move: 'f3', comments: [] },
        { move: 'e6', comments: [] },
        { move_number: 2, move: 'g4', nags: [ '??', '∓', '$5', '∞' ], comments: [] },
        { move: 'Qh4#', comments: [] }
    ],
    result: '0-1'
}

NOTE: This pull request does slightly change the behavior of the parser:

Parsing the move 2. g4?? $2 currently produces:

{ move_number: 2, move: 'g4??', nags: ['$2'], comments: []}

After this commit the new result would be:

{ move_number: 2, move: 'g4', nags: ['??', '$2']}

I've made this a draft pull request in case there are objections to this behavior change.