LibWeb: Implement <script src> support for synchronous scripts

Scripts loaded in this way will block the parser until they finish
executing. This means that they see the DOM before the whole document
has been fully parsed. This is all normal, of course.

To make this work, I changed the way we notify DOM nodes about tree
insertion. The inserted_into() callbacks are now incrementally invoked
during parse, as each node is appended to its parent.

To accomodate inline scripts and inline style sheets, we now also have
a children_changed() callback which is invoked on any parent when it
has children added/removed.

Libraries/LibWeb/DOM/HTMLScriptElement.h

@@ -36,6 +36,7 @@ public:
     virtual ~HTMLScriptElement() override;
 
     virtual void inserted_into(Node&) override;
+    virtual void children_changed() override;
 };
 
 }