fix(nextjs): Restore tree shaking capabilities

[lforst]

We messed up in #6592 and destroyed tree shaking capabilities for webpack by entirely removing the sideEffects field in the package.json. This PR adds it back so tree shaking should be possible again.

Hopefully fixes#7680

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.66 KB (0%)
@sentry/browser - ES5 CDN Bundle (minified)	64.52 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.2 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	56.89 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.57 KB (0%)
@sentry/browser - Webpack (minified)	72.11 KB (0%)
@sentry/react - Webpack (gzipped + minified)	21.59 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	52 KB (-0.18% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.26 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.45 KB (0%)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.78 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	38.86 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	63.5 KB (0%)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	56.53 KB (0%)

[AbhiPrasad]

Can we test this to make sure we don't regress?

Also did you validate with a local app? Can you show before/after in the PR?

[lforst]

Can we test this to make sure we don't regress?

We already have tests in place for this! E2E tests would fail if this would eliminate side effects somehow.

Also did you validate with a local app? Can you show before/after in the PR?

Well, I just tried it out locally, and turns out replay does properly tree shake away in my Next.js 13 test application, regardless of this change. Maybe it is related to older webpack versions.

I am still going to go forward with this because it is more correct this way.

[rchl]

I've crated somewhat relevant #7713