Build for Node 18

[ahuth]

Raise the min supported Node version from 12 to 18.

[jeremiah-clothier]

Would adding the engines property be worthwhile? I know it's a documented property but not sure what the dev experience of the property actually is and how it would change npm install behavior for someone with node < 18

https://docs.npmjs.com/cli/v10/configuring-npm/package-json#engines

[ahuth]

I don't love putting engines in, because it might work on other versions of node (depends on what features I used). Figure it's best to document what version is supported, but let people try with whatever version they want.

[ahuth]

Although technically it would still work with engines unless someone configures engines-strict