build: Build core & utils into a single output file

[mydea]

This may speed up usage, as it avoids some potentially expensive require/import calls.

Let's try how this affects test run times first...

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	21.02 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	65.66 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.83 KB (+1.39% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	58.67 KB (+0.96% 🔺)
@sentry/browser - Webpack (gzipped + minified)	21.77 KB (+2.84% 🔺)
@sentry/browser - Webpack (minified)	70.18 KB (+1.62% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.8 KB (+2.88% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	48.53 KB (-1.04% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.59 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	27.04 KB (+0.8% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	47.13 KB (+1.87% 🔺)
@sentry/replay - Webpack (gzipped + minified)	40.14 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	65.33 KB (+0.32% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	58.36 KB (+0.44% 🔺)