The initial system to send to ChatGPT API. This can be used to set the initial context of the conversation, and
instruct ChatGPT to use a specific persona.

* Method: `withSystemMessage(systemMessage)`
* Argument Type: `string`
* Required: `false`
* Usage:

```js
const adapter = createUnsafeChatAdapter()
    .withSystemMessage('I want you to act as a customer support agent.');
```
