[cssom-1] Define `CSSRule.cssText` with `readonly`

Since https://github.com/w3c/csswg-drafts/commit/9f8435ab9f8b2a6ab76e54d2cfd06bec4526ea5a.

  > ```
  > interface CSSRule { 
  >   attribute CSSOMString cssText;
  >   // ...
  > }
  > ```
  > On setting the `cssText` attribute must do nothing.

Would it be different / a problem if [`CSSRule.cssText`](https://w3c.github.io/csswg-drafts/cssom-1/#cssrule) were defined with `readonly`?