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.

> * Property: `systemMessage`
> * Type: `string`
> * Usage:

```tsx
const adapter = useUnsafeChatAdapter({
    systemMessage: 'I want you to act as a customer support agent.'
});
```
