script: Implement deprecated CSSStyleSheet members (#36313)

Implements `rules`, `addRule()` and `removeRule()` for `CSSStyleSheet`.
https://drafts.csswg.org/cssom/#legacy-css-style-sheet-members

This is part of #36162

Testing:
- `/css/css-cascade/at-scope-parsing.html`
- `/css/css-conditional/at-supports-whitespace.html`
- `/css/css-nesting/invalidation-004.html`
- `/css/css-nesting/parsing.html`
- `/css/css-nesting/serialize-group-rules-with-decls.html`
- `/css/css-syntax/custom-property-rule-ambiguity.html`
- `/css/css-syntax/invalid-nested-rules.html`
- `/css/css-syntax/trailing-braces.html`
- `/css/css-syntax/var-with-blocks.html`
- `/css/css-transitions/parsing/starting-style-parsing.html`
- `/css/cssom/CSSStyleSheet.html`
- `/css/cssom/idlharness.html`
- `/css/cssom/insertRule-across-context.html`

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

components/script_bindings/webidls/CSSStyleSheet.webidl

@@ -10,3 +10,14 @@ interface CSSStyleSheet : StyleSheet {
   [Throws] unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
   [Throws] undefined deleteRule(unsigned long index);
 };
+
+// https://drafts.csswg.org/cssom/#legacy-css-style-sheet-members
+partial interface CSSStyleSheet {
+  [Throws, SameObject] readonly attribute CSSRuleList rules;
+  [Throws] long addRule(
+      optional DOMString selector = "undefined",
+      optional DOMString style = "undefined",
+      optional unsigned long index
+  );
+  [Throws] undefined removeRule(optional unsigned long index = 0);
+};