LibWeb/CSS: Clear child CSS rules' caches too

If a rule gets its caches cleared because it's moved in the OM, then its
child rules' caches are likely invalid and need clearing too.

Assuming that caches only point "upwards", this will correctly clear
them all. For the time being that will be true.

Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/cssom.txt

@@ -6,7 +6,8 @@ Rerun
 
 Found 13 tests
 
-13 Fail
+1 Pass
+12 Fail
 Details
 Result	Test Name	MessageFail	CSSStyleRule is a CSSGroupingRule	
 Fail	Simple CSSOM manipulation of subrules	
@@ -20,4 +21,4 @@ Fail	Simple CSSOM manipulation of subrules 7
 Fail	Simple CSSOM manipulation of subrules 8	
 Fail	Simple CSSOM manipulation of subrules 9	
 Fail	Simple CSSOM manipulation of subrules 10	
-Fail	Mutating the selectorText of outer rule invalidates inner rules	
+Pass	Mutating the selectorText of outer rule invalidates inner rules	

Userland/Libraries/LibWeb/CSS/CSSGroupingRule.cpp

@@ -34,6 +34,13 @@ void CSSGroupingRule::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_rules);
 }
 
+void CSSGroupingRule::clear_caches()
+{
+    Base::clear_caches();
+    for (auto& rule : *m_rules)
+        rule->clear_caches();
+}
+
 WebIDL::ExceptionOr<u32> CSSGroupingRule::insert_rule(StringView rule, u32 index)
 {
     TRY(m_rules->insert_a_css_rule(rule, index));

Userland/Libraries/LibWeb/CSS/CSSGroupingRule.h

@@ -35,6 +35,7 @@ class CSSGroupingRule : public CSSRule {
 
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
+    virtual void clear_caches() override;
 
 private:
     JS::NonnullGCPtr<CSSRuleList> m_rules;

Userland/Libraries/LibWeb/CSS/CSSRule.h

@@ -58,13 +58,13 @@ class CSSRule : public Bindings::PlatformObject {
     // https://drafts.csswg.org/cssom-1/#serialize-a-css-rule
     virtual String serialized() const = 0;
 
+    virtual void clear_caches();
+
 protected:
     explicit CSSRule(JS::Realm&, Type);
 
     virtual void visit_edges(Cell::Visitor&) override;
 
-    virtual void clear_caches();
-
     [[nodiscard]] FlyString const& parent_layer_internal_qualified_name() const
     {
         if (!m_cached_layer_name.has_value())