map2<R>  method 
Maps a parsed Record to R using the provided callback, see
MapParserExtension.map for details.
Implementation
@useResult
Parser<R> map2<R>(
  R Function(T1, T2) callback, {
  bool hasSideEffects = false,
}) => map((record) => record.map(callback), hasSideEffects: hasSideEffects);