[MessageHandler] Replace the internal `isReply` property, as sent when Promise callbacks are used, with enumeration values
[MessageHandler] Replace the internal `isReply` property, as sent when Promise callbacks are used, with enumeration values

Given that the `isReply` property is an internal implementation detail, changing its type shouldn't be a problem. Note that by directly indicating if either data or an Error is sent, it's no longer necessary to use `in` when handling the callback.

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