LibWeb: Start working on DOM event support

This patch adds the EventTarget class and makes Node inherit from it.

You can register event listeners on an EventTarget, and when you call
dispatch_event() on it, the event listeners will get invoked.

An event listener is basically a wrapper around a JS::Function*.

This is pretty far from how DOM events should eventually work, but it's
a place to start and we'll build more on top of this. :^)

Libraries/LibWeb/Parser/HTMLParser.cpp

@@ -378,6 +378,8 @@ RefPtr<Document> parse_html_document(const StringView& html, const URL& url)
     };
     fire_insertion_callbacks(document);
 
+    document->dispatch_event("DOMContentLoaded");
+
     return document;
 }