fix(nextjs): Fix types in config code

[lobsterkatie]

This cleans up the types we use in withSentryConfig and friends. Three specific changes:

• When a user passes their nextjs config to nextjs, they're free to specify as few or as many of the options as they like. Nextjs config objects provided directly by nextjs, on the other hand, will always have certain properties, because they will have been filled in with defaults if not specified by the user. This differentiates between these two cases, through the use of the Partial< ... > utility type when describing config provided by the user.

• In order to play more nicely with @sentry/webpack-plugin and its types, this uses the official WebpackPluginInstance type from webpack itself. This requires including webpack as a dependency, in this case a peer dependency, since we're only using it for its types and therefore don't actually care if it's installed or not. (Given nextjs's reliance on it, you might be asking yourself how a nextjs user would ever not have it already installed, and the answer is that nextjs ships with a precompiled version and therefore doesn't include it as anything other than a dev dependency.)

• Given that we still support webpack 4, this ramps our types dev dependency down to the lowest version which includes the compatibility type WebpackPluginInstance.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	22.34 KB (0%)
@sentry/browser - Webpack	23.31 KB (0%)
@sentry/react - Webpack	23.34 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	29.79 KB (+0.01% 🔺)

[kamilogorek]

Looks good and all tests, including integration tests for next.js are passing, so 👍

[iker-barriocanal]

🚀