LibJS: Move number_format_relevant_extension_keys to Intl.NumberFormat

This method represents the Intl.NumberFormat's [[RelevantExtensionKeys]]
internal slot, so it makes more sense for this to be directly in the
class itself.

Userland/Libraries/LibJS/Runtime/Intl/NumberFormat.h

@@ -70,6 +70,8 @@ public:
         ExceptZero,
     };
 
+    static Vector<StringView> const& relevant_extension_keys(); // [[RelevantExtensionKeys]]
+
     NumberFormat(Object& prototype);
     virtual ~NumberFormat() override = default;