Support ES modules in `next.config.js`

[ehmicky]

ES modules are only supported in Node.js 12, and import() must be used instead of require().
We currently use require() because we support Node 10.

https://github.com/netlify/netlify-plugin-nextjs/blob/59ee2bbb2ffb971e964a07245eaa478ba1c07cbd/package.json#L55

It seems like some users are trying to use ES modules in their next.config.js, which currently fails.
Bugsnag errors here, here and there.

This will be a growing problem in 3 months (April 2021) when Node 10 becomes officially unsupported.

[ascorbic]

This is an upstream issue: Next.js doesn't support it either. See vercel/next.js#9607
This should no longer show up on Bugsnag though, as it's a user build failure now.