LibWeb: Add a basic implementation of Document.createEvent()

This is a legacy function providing a way of constructing events without
using their constructors exposed on the global object.
We don't have many of the events it supports yet, nor can we throw a
DOMException from it, so that's two FIXMEs for later.

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -30,6 +30,7 @@ interface Document : Node {
     Text createTextNode(DOMString data);
     Comment createComment(DOMString data);
     Range createRange();
+    Event createEvent(DOMString interface);
 
     [CEReactions, ImplementedAs=adopt_node_binding] Node adoptNode(Node node);