test(loader): Improve loader tests & update loader

[mydea]

This updates the loader based on this PR: getsentry/sentry#58070
and also actually fixes/improves the loader tests by avoiding importing @sentry/browser. This is not needed and actually obfuscates the "real"/"normal" behavior (just look at the generated dist folder for a test.

Instead, I added an eslint rule to allow Sentry as a global for these cases, and just access them directly. This also uncovered an incorrect test, which I adjusted so it works. I also added a new test to ensure custom config works.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	84 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.6 KB (0%)
@sentry/browser - Webpack (gzipped)	21.77 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	80.5 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.54 KB (-0.01% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.71 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	245.65 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	85.63 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	61.55 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	32.66 KB (0%)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	84.07 KB (0%)
@sentry/react - Webpack (gzipped)	21.81 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	102.21 KB (0%)
@sentry/nextjs Client - Webpack (gzipped)	49.67 KB (0%)