LibWeb: Move ARIA-related code into the Web::ARIA namespace

ARIA has its own spec and is not part of the DOM spec, which is what the
Web::DOM namespace is for (https://www.w3.org/TR/wai-aria-1.2/).

This allows us to stay closer to the spec with function names and don't
have to add the word "ARIA" to identifiers constantly - the namespace
now provides that clarity.

Userland/Libraries/LibWeb/HTML/HTMLAreaElement.h

@@ -42,7 +42,7 @@ private:
         queue_an_element_task(source, move(steps));
     }
 
-    virtual Optional<DOM::ARIARoles::Role> default_role() const override;
+    virtual Optional<ARIA::Role> default_role() const override;
 };
 
 }