LibWeb: Update focusing spec steps

Update a couple of focus-related spec steps and their implementations.
The most relevant change is that we no longer allow focusing on elements
that return false for `->is_focusable()`, which necessitates fixing a
broken test that tried to `.focus()` on `<div>`s that were not
focusable. That test's output now more accurately reflects the expected
outcome as seen in other browsers.

Libraries/LibWeb/HTML/HTMLDialogElement.h

@@ -23,6 +23,9 @@ public:
 
     virtual void removed_from(Node* old_parent, Node& old_root) override;
 
+    // ^EventTarget
+    virtual bool is_focusable() const override { return true; }
+
     String return_value() const;
     void set_return_value(String);