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/CSSRule.h

@@ -47,6 +47,9 @@ public:
     virtual StringView class_name() const = 0;
     virtual Type type() const = 0;
 
+    template<typename T>
+    bool fast_is() const = delete;
+
 private:
 };