fix(nextjs): Mark multiplexer targets as entrypoints

[lforst]

Similar to #6905, https://github.com/getsentry/sentry-javascript/pull/6817/files#diff-5598248a0f2280565998f93876df482b2040cb761cc510b3bd2c4dfa5824dca8L103 started tree shaking away the exports from the files we were targeting with the multiplexer, causing some imports not to be available. This PR fixes this by marking the targets as entrypoints.

Fixes #6915

(I am deeply questioning our test suite at this point)

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.83 KB (0%)
@sentry/browser - ES5 CDN Bundle (minified)	61.48 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.5 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	54.79 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.22 KB (0%)
@sentry/browser - Webpack (minified)	66.19 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.25 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.54 KB (+0.11% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.76 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.03 KB (0%)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.2 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	37.92 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	60.55 KB (0%)