ref: Allow withSentryConfig to accept async config function

[kamilogorek]

Modified one template to use the async function instead, as adding an additional variant to an already bloated integration test runner felt meh.

Note that type from #7444 is incorrect.
It should be:

const nextConfig = async () => {
  /** @type {import('next').NextConfig} */
  return {};
}

and not:

/** @type {import('next').NextConfig} */
const nextConfig = async () => {
  return {};
}

Note #2: async function handling works for next >= 12.1 only: https://nextjs.org/docs/app/api-reference/next-config-js/pageExtensions

Verified the behavior locally using provided sample config from the original issue.

Fixes #7444