Replace node-fetch with fetch from Node 18

[jonnitto]

Hi David First of all, thank you for your package!

With Node 18, fetch is native built in. I know, this would be breaking change to replace it, but as Node 18 is now the current LTS, it would be an option to think about it…

[davext]

I removed the dependency and made it compatible with Cloudflare workers! Here: https://www.npmjs.com/package/cf-worker-mautic