feat: NextJS initialization config

[iker-barriocanal]

This PR focuses on the initialization of the NextJS SDK.

Main changes

1. Added entry points in package.json for browser and node environments.
2. options.ts has been refactored. The NextjsOptions interface and MetadataBuilder are now in separate files in /utils, instead of being in the same one.
3. The SDK is not always initialized. utils/initDecider is responsible for this, and by default only initializes the SDK when it's run in production; although the initialization in non-production environments can be forced by setting dev: true (see example below).

Why is the SDK by default only initialized in production?

The code that is being run in development mode is not the same as the one in production mode. NextJS requires building the app before running it in production mode (docs), which besides performing some optimizations, does some checks (e.g. compilation errors). Some of these errors, which won't ever be run in production due to the building phase, can actually be run in development; they break the app and Sentry doesn't deal with them (at the moment). Thus, the SDK is only initialized by default in production environments.

However, some users might be interested in running Sentry in development. The SDK initialization can be forced by setting dev to true (false by default) in init, as follows:

import * as Sentry from '@sentry/nextjs';

Sentry.init({
  dsn: '__DSN__',
  dev: true,
  // ...
});

This will initialize the SDK in non-production modes. Note, however, that this doesn't change the actual behavior; so there might be cases where what described above happens. If this is the case: try to build the app, fix the errors, and once it's built, run it again.

[HazAT]

I think this PR contains the changes of the branch or it was not based of the feat/next-js branch. Something is odd with the diff.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	20.27 KB (+0.02% 🔺)
@sentry/browser - Webpack	21.18 KB (0%)
@sentry/react - Webpack	21.2 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	27.37 KB (+0.02% 🔺)