LibWeb: Add a non-DeprecatedString version of Element::get_attribute

Renaming the DeprecatedString version of this function to
Element::get_deprecated_attribute.

While performing this rename, port over functions where it is trivial to
do so to the Optional<String> version of this function.

Userland/Libraries/LibWeb/HTML/HTMLFormElement.cpp

@@ -394,7 +394,7 @@ static bool is_form_control(DOM::Element const& element)
     }
 
     if (is<HTMLInputElement>(element)
-        && !element.get_attribute(HTML::AttributeNames::type).equals_ignoring_ascii_case("image"sv)) {
+        && !element.deprecated_get_attribute(HTML::AttributeNames::type).equals_ignoring_ascii_case("image"sv)) {
         return true;
     }