fix(nextjs): Fix typing issue with withSentryConfig and NextConfig

[timfish]

Closes #4560

This was super nasty to fix!

I managed to fix the reported null issue and a couple of other minor differences too, I finally get stuck on this horror:

Argument of type 'NextConfig' is not assignable to parameter of type 'NextConfigObjectWithSentry | NextConfigFunctionWithSentry | undefined'.
  Type 'NextConfig' is not assignable to type 'NextConfigObjectWithSentry'.
    Type 'NextConfig' is not assignable to type '{ webpack?: WebpackConfigFunction | null | undefined; target?: "server" | "experimental-serverless-trace" | undefined; distDir?: string | undefined; basePath?: string | undefined; publicRuntimeConfig?: { [key: string]: unknown; } | undefined; pageExtensions?: string[] | undefined; }'.
      Types of property 'webpack' are incompatible.
        Type 'NextJsWebpackConfig | null | undefined' is not assignable to type 'WebpackConfigFunction | null | undefined'.
          Type 'NextJsWebpackConfig' is not assignable to type 'WebpackConfigFunction'.
            Types of parameters 'context' and 'options' are incompatible.
              Type 'BuildContext' is not assignable to type 'WebpackConfigContext'.
                Types of property 'config' are incompatible.
                  Type 'NextConfigObject' is not assignable to type 'NextConfigComplete'.
                    Type 'NextConfigObject' is missing the following properties from type 'Required<NextConfig>': exportPathMap, i18n, eslint, typescript, and 33 more.  ts(2345)

The issue is that NextJsWebpackConfig has the property config: NextConfigComplete and NextConfigComplete is Required<NextConfig>. I could go through and add the 33 missing properties to the vendored NextConfigObject but it will get broken as soon as new properties get added.

I spent an age trying to work out how to fix this and it basically comes down to the fact that Required is evil and makes creating vendored types close to impossible, especially when you rely on the detail of those vendored types in your own code.

I resigned myself to making the smallest change somewhere to any and found that I could do that with the config in BuildContext. So we don't lose type safety in the Sentry code, I had to add config as NextConfigObject where appropriate.

This PR also adds a test:types script which runs after the integration tests and ensures that this doesn't get broken in the future. It has a package.json that means that we can test against the next v12 types. I did try updating @sentry/nextjs to devDepend on v12 but it threw up a few errors and I wasn't convinced this was a good idea.

[lforst]

Thank you Tim for fixing this! This looks good to me.

[lforst]

E2E tests are failing because they're requiring secrets we don't have in forks. I opened #5986 to fix this.