Gemini system prompt isn't sent as system prompt

We currently send the system prompt as a user message:
https://github.com/token-js/token.js/blob/9074900a94cf97ef501ee989df416bfd8d9cb6b8/src/handlers/gemini.ts#L110C1-L112C101

But system prompts are officially supported on the gemini sdk:
https://ai.google.dev/gemini-api/docs/text-generation?lang=node#system-instructions