LibWeb: Add more fast_is<T> helpers for DOM::Node subclasses

Author: https://github.com/awesomekling
Commit: aecb144df0
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4395

Libraries/LibWeb/HTML/HTMLMediaElement.h

@@ -182,6 +182,8 @@ protected:
 private:
     friend SourceElementSelector;
 
+    virtual bool is_html_media_element() const final { return true; }
+
     struct EntireResource { };
     using ByteRange = Variant<EntireResource>; // FIXME: This will need to include "until end" and an actual byte range.
 
@@ -324,3 +326,8 @@ private:
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLMediaElement>() const { return is_html_media_element(); }
+}