LibWeb: Create HTMLInputElement UA shadow tree when inserted into DOM

Previously, we were creating a user-agent shadow tree when constructing
a layout tree. This meant that we did DOM manipulation (and consequently
style invalidation) during layout tree construction, which made things
very hard to reason about in Layout::TreeBuilder.

Simply everything by simply creating the UA shadow tree when the input
element inserted into a parent node instead.

Userland/Libraries/LibWeb/HTML/HTMLInputElement.h

@@ -100,6 +100,8 @@ public:
     // https://html.spec.whatwg.org/multipage/forms.html#category-label
     virtual bool is_labelable() const override { return type_state() != TypeAttributeState::Hidden; }
 
+    virtual void inserted() override;
+
 private:
     // ^DOM::EventTarget
     virtual void did_receive_focus() override;