Fixes jsdom/jsdom#3985, at least a big part of it.

During set cssText the library calls setProperty many times but wants to fire the onChange callback only once at the end. That's what the this._setInProgress field is for. But despite _setInProgress being true, the get cssText getter is called many times: to get the originalText, and because of suboptimal order of conditions.

This patch helps avoid these unneeded and expensive get cssText calls. They are expensive because they serialize the AST structure to a string.

I've been testing this on a little JSDOM benchmark script:

console.time('create-100-divs');
for (let i = 0; i < 100; i++) {
  const div = document.createElement('div');
  div.style.cssText = 'color: blue; background-color: white; padding: 10px; border: 1px solid black;';
  div.textContent = `Div #${i + 1}`;
  document.body.appendChild(div);
}
console.timeEnd('create-100-divs');

Before this PR, it takes 230ms to execute, after this PR it's 130ms, almost 2x improvement.

I think there are going to be even more optimization opportunities. If I remove the div.style.cssText assignment, the whole loop runs in 1ms.