function isDirectionRTL(
  str: string,
  strongFallback: ?BidiDirection,
): boolean {
  return getDirection(str, strongFallback) === UnicodeBidiDirection.RTL;
}
Returns true if getDirection(arguments...) returns RTL.

@param str             A text block; e.g. paragraph, table cell
@param strongFallback  Fallback direction, used if no strong direction
                       detected for the block (default = global direction)
@return                True if the resolved direction is RTL