fix: ensure priority is set before calling the onchange callback

[LeahHirst]

fixes jsdom/jsdom#2649

The onChange callback is called prematurely when invoking setProperty with an important priority. This results in elements wrongly being assigned an inline style attribute value without !important. Setting multiple important properties in jsdom results in the last one assigned missing !important:

const el = document.createElement('div');
el.style.setProperty('opacity', '0', 'important');
el.style.setProperty('width', '0', 'important');
el.getAttribute('style'); // `opacity: 0 !important; width: 0;`