LibJS: Implement private identifiers in optional chains

Author: https://github.com/davidot
Commit: 2d48529073
Pull-request: https://github.com/SerenityOS/serenity/pull/10470
Issue: https://github.com/SerenityOS/serenity/issues/7044
Issue: https://github.com/SerenityOS/serenity/issues/8574
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibJS/AST.h

@@ -1590,8 +1590,12 @@ public:
         NonnullRefPtr<Identifier> identifier;
         Mode mode;
     };
+    struct PrivateMemberReference {
+        NonnullRefPtr<PrivateIdentifier> private_identifier;
+        Mode mode;
+    };
 
-    using Reference = Variant<Call, ComputedReference, MemberReference>;
+    using Reference = Variant<Call, ComputedReference, MemberReference, PrivateMemberReference>;
 
     OptionalChain(SourceRange source_range, NonnullRefPtr<Expression> base, Vector<Reference> references)
         : Expression(source_range)