LibWeb: Make FormAssociatedElement inherit from HTMLElement

The new event target implementation requires us to downcast an
EventTarget to a FormAssociatedElement to check if the current Element
EventTarget has a form owner to setup a with scope for the form owner.

This also makes all form associated elements inherit from
FormAssociatedElement where it was previously missing.

https://html.spec.whatwg.org/#form-associated-element

Userland/Libraries/LibWeb/HTML/HTMLSelectElement.cpp

@@ -11,7 +11,7 @@
 namespace Web::HTML {
 
 HTMLSelectElement::HTMLSelectElement(DOM::Document& document, QualifiedName qualified_name)
-    : HTMLElement(document, move(qualified_name))
+    : FormAssociatedElement(document, move(qualified_name))
 {
 }