fix(next): Inject SDK in dev mode

[lforst]

Fixes #6321

In #6206 we switched up our logic not to always include the SDK on /app route. This broke the injection mechanism in dev mode where _app is the only entry point that is always hit. In dev mode, pages defined by the user are not considered entry points so they were skipped by our injection logic.

This PR fixes this by always injecting the SDK into _app in dev mode. This is probably not optimal because it doesn't really reflect what is happening when a Next.js is built using prod mode but it's as close as we can get with reasonable effort.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.57 KB (-0.03% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.59 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.36 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	54.15 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.12 KB (0%)
@sentry/browser - Webpack (minified)	65.75 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.14 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.97 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.41 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.85 KB (+0.01% 🔺)
@sentry/replay index.js	37.2 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	37.17 KB (0%)
@sentry/replay - Webpack (minified)	124.99 KB (0%)

[lobsterkatie]

LGTM!