parse static method
Implementation
static CSSFunction? parse(String? s) {
  CSSFunction? cssF = CSSCalc.parse(s);
  if (cssF != null) return cssF;
  cssF = CSSMax.parse(s);
  if (cssF != null) return cssF;
  cssF = CSSMin.parse(s);
  if (cssF != null) return cssF;
  return null;
}