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/HTMLAnchorElement.h

@@ -53,7 +53,7 @@ private:
         queue_an_element_task(source, move(steps));
     }
 
-    virtual DeprecatedFlyString default_role() const override;
+    virtual Optional<DOM::ARIARoles::Role> default_role() const override;
 };
 
 }