fix(nextjs): correct withSentryConfig return type

[seanparmelee]

In #6291, withSentryConfig was updated to always return a function so this PR updates the return value type to no longer include NextConfigObject.

[lforst]

Hi, thanks for opening this PR. The return type was intentionally kept broad because we might want to change this again in the future without introducing a breaking change.

[seanparmelee]

Thanks for the consideration. FWIW, #6291 was a bit of a breaking change for us because we were expecting withSentryConfig to return an object since we're passing in an object and TS didn't catch it because it thinks an object can still be returned.

[lforst]

TS should complain if you don't handle the case where an object is returned. Technically this was not a breaking change.

[seanparmelee]

It might be helpful if I provide a bit more background on how we encountered this.
We're using next-transpile-modules similar to this:

// next.config.js
const withTM = require('next-transpile-modules')([...]);
const { withSentryConfig } = require('@sentry/nextjs');

...

module.exports = withTM(
  withSentryConfig({
    ...
  }, {...})
);

We're currently using @sentry/nextjs 6.19.7 and attempted to upgrade to 7.29.0 when we noticed our landing page stopped loading due to what looked like a GraphQL error. We looked through the migration guide to ensure we didn't overlook any required changed, but didn't see anything that applied to our usage. Stepping through the code, we discovered our Next runtime config was empty (resulting in an incorrect GraphQL URL being used and ultimately causing the error we were seeing). This is what lead us to look into what changed with withSentryConfig and looking into next-transpile-modules to find that it does not support passing the next config as a function. If we pin @sentry/nextjs to 7.21.0, the landing page loads without issue.

Fortunately, we realized this particular app longer needs next-transpile-modules so we were able to remove it and have everything work again with @sentry/nextjs 7.29.0. If we needed to keep this plugin around, it appears that swapping the withTM and withSentryConfig calls also works, even though the docs mention withTM should be the outermost plugin. In either case, this was a bit of a surprise to us (and might be for others that are also using next-transpile-modules).