LibWeb: Add fast_is<T> for CSSRule subclasses

Since we already have a type enum for these, let's use it to make
is<T> bypass dynamic_cast for CSS rules.

These were often near the top of random browser profiles.

Userland/Libraries/LibWeb/CSS/CSSImportRule.h

@@ -60,4 +60,7 @@ private:
     RefPtr<CSSStyleSheet> m_style_sheet;
 };
 
+template<>
+inline bool CSSRule::fast_is<CSSImportRule>() const { return type() == CSSRule::Type::Import; }
+
 }