LibWeb: Start work towards modern CSS "display" values

Until now, we've internally thought of the CSS "display" property as a
single-value property. In practice, "display" is a much more complex
property that comes in a number of configurations.

The most interesting one is the two-part format that describes the
outside and inside behavior of a box. Switching our own internal
representation towards this model will allow for much cleaner
abstractions around layout and the various formatting contexts.

Note that we don't *parse* two-part "display" yet, this is only about
changing the internal representation of the property.

Spec: https://drafts.csswg.org/css-display

Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp

@@ -47,7 +47,7 @@ RefPtr<Layout::Node> HTMLInputElement::create_layout_node()
         return nullptr;
 
     auto style = document().style_computer().compute_style(*this);
-    if (style->display() == CSS::Display::None)
+    if (style->display().is_none())
         return nullptr;
 
     if (type().equals_ignoring_case("submit") || type().equals_ignoring_case("button"))