src/common/util.js/skip
function skip(chars) {
  return (text, index, opts) => {
    const backwards = opts && opts.backwards;

    // Allow `skip` functions to be threaded together without having
    // to check for failures (did someone say monads?).
    if (index === false) {
      return false;
    }

    const { length } = text;
    let cursor = index;
    while (cursor >= 0 && cursor < length) {
      const c = text.charAt(cursor);
      if (chars instanceof RegExp) {
        if (!chars.test(c)) {
          return cursor;
        }
      } else if (!chars.includes(c)) {
        return cursor;
      }

      backwards ? cursor-- : cursor++;
    }

    if (cursor === -1 || cursor === length) {
      // If we reached the beginning or end of the file, return the
      // out-of-bounds cursor. It's up to the caller to handle this
      // correctly. We don't want to indicate `false` though if it
      // actually skipped valid characters.
      return cursor;
    }
    return false;
  };
}