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/HTMLTableCellElement.cpp

@@ -74,7 +74,7 @@ void HTMLTableCellElement::set_row_span(unsigned int value)
     MUST(set_attribute(HTML::AttributeNames::rowspan, DeprecatedString::number(value)));
 }
 
-DeprecatedFlyString HTMLTableCellElement::default_role() const
+Optional<DOM::ARIARoles::Role> HTMLTableCellElement::default_role() const
 {
     // TODO: For td:
     //       role=cell if the ancestor table element is exposed as a role=table