function firstStrongCharDir(str: string): BidiDirection {
  const strongChar = firstStrongChar(str);
  if (strongChar == null) {
    return UnicodeBidiDirection.NEUTRAL;
  }
  return REGEX_RTL.exec(strongChar)
    ? UnicodeBidiDirection.RTL
    : UnicodeBidiDirection.LTR;
}
Returns the direction of a block of text, based on the direction of its
first strong character (has Bidi_Class value of L, R, or AL).

@param str  A text block; e.g. paragraph, table cell, tag
@return     The resolved direction