function firstStrongChar(str: string): ?string {
  const match = REGEX_STRONG.exec(str);
  return match == null ? null : match[0];
}
Returns the first strong character (has Bidi_Class value of L, R, or AL).

@param str  A text block; e.g. paragraph, table cell, tag
@return     A character with strong bidi direction, or null if not found