Environment detection broken in Node 18

[lukastaegert]

This package uses

if (typeof fetch === "function") {
  // ...
}

to detect if it is running in a browser environment. This no longer works in latest Node as fetch is now available as a regular Node global. I currently use delete global.fetch in my code to work around this, but it would be preferable if it uses a better environment sniffing algorithm, e.g. by checking for for Node-only globals.

[eemeli]

Duplicate of #349.