support for '\unicode'

[lopo12123]

This library does not support rendering \unicode{xxx} into unicode characters, you can match such formulas before parsing and replace them with unicode characters to display

Here I use regular expressions to filter and replace, it should be able to support \unicode{xxx}, hope you can add it if you have time

/// replace all unicode directives before parsing
///
/// rule of '\unicode{xxx}'
/// - \unicode{ [^x][0-9a-zA-Z]* }  -- decimal
/// - \unicode{ x[0-9a-zA-Z]* } -- hexadecimal
String unicodeFilter(String raw) {
  var reg = RegExp(
    r'\${1,2}\\unicode\{(?<isHex>x)?(?<val>[0-9a-eA-E]*)\}\${1,2}',
    dotAll: true,
  );

  String replaced = '';
  int p = 0;

  reg.allMatches(raw).forEach((part) {
    if (part.start > p) {
      replaced += raw.substring(p, part.start);
    }

    bool isHex = part.namedGroup('isHex') == 'x';
    int? v = int.tryParse(part.namedGroup('val') ?? '', radix: isHex ? 16 : 10);

    if (v != null) replaced += String.fromCharCode(v);

    p = part.end;
  });

  return replaced;
}