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/HTMLImageElement.h

@@ -43,7 +43,7 @@ public:
     // https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-complete
     bool complete() const;
 
-    virtual DeprecatedFlyString default_role() const override;
+    virtual Optional<DOM::ARIARoles::Role> default_role() const override;
 
 private:
     HTMLImageElement(DOM::Document&, DOM::QualifiedName);