Node minimum version

[XaveScor]

Vitest supports node>=18 only, but vitest-fetch-mock supports node 14 in package.json.

https://github.com/vitest-dev/vitest/blob/d7b0387125da9c2882b12a1f63cc693dc049b15c/package.json#L9

I think, we can safely up the minimum supported Node version up to v18. It allows the lib to use the native fetch instead of cross-fetch polyfill.

[IanVS]

Ah, yes, my intention was to support node 18+, I just didn't notice I had an engines field. Would you like to submit a PR?

[XaveScor]

I'll make a PR