LibWeb: Replace ARIA role static FlyStrings with an enum

This replaces the FlyStrings for ARIA roles that were constructed in
a [[gnu::constructor]] with a single enum. I came across this as the
DOM inspector was crashing due to a null FlyString for an ARIA role.

After fixing that, I was confused as to why these roles were not an
enum. Looking at the spec there's a fixed list of roles and switching
from references to static strings to an enum was pretty much an
exercise in find and replace :).

No functional changes (outside of fixing the mentioned crash).

Userland/Libraries/LibWeb/HTML/HTMLHtmlElement.h

@@ -6,7 +6,7 @@
 
 #pragma once
 
-#include <LibWeb/DOM/ARIARoleNames.h>
+#include <LibWeb/DOM/ARIARoles.h>
 #include <LibWeb/HTML/HTMLElement.h>
 
 namespace Web::HTML {
@@ -20,7 +20,7 @@ public:
     bool should_use_body_background_properties() const;
 
     // https://www.w3.org/TR/html-aria/#el-html
-    virtual DeprecatedFlyString default_role() const override { return DOM::ARIARoleNames::document; }
+    virtual Optional<DOM::ARIARoles::Role> default_role() const override { return DOM::ARIARoles::Role::document; }
 
 private:
     HTMLHtmlElement(DOM::Document&, DOM::QualifiedName);