fix(sveltekit): Properly await sourcemaps flattening

[Lms24]

As pointed out in #10589, we didn't properly await sourcemaps flattening via sorcery before proceeding to upload them. The reason is that the async callbacks in forEach weren't awaited. A for loop is the better approach here. Wondering if we should lint against async forEach callbacks.
This behaviour could have caused various inconsistencies. My suspicion is that the timing worked well enough in most cases but we definitely want to properly await this step.

Thanks to @MSDev201 for bringing this up!

Unfortunately this likely won't fix #10589 as a whole :(

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	78.1 KB (-0.08% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	69.31 KB (-0.09% 🔽)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	73.25 KB (-0.09% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.95 KB (-0.1% 🔽)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	33.27 KB (-0.25% 🔽)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	33.13 KB (-0.25% 🔽)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.15 KB (-0.15% 🔽)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.15 KB (-0.19% 🔽)
@sentry/browser - Webpack (gzipped)	22.4 KB (-0.35% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	76.08 KB (-0.15% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.62 KB (-0.14% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.37 KB (-0.32% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.46 KB (-0.41% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	213.23 KB (-0.21% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	101.03 KB (-0.44% 🔽)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	73.42 KB (-0.6% 🔽)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.54 KB (-0.28% 🔽)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.71 KB (-0.09% 🔽)
@sentry/react - Webpack (gzipped)	22.45 KB (-0.3% 🔽)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	87.45 KB (-0.1% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	50.35 KB (-0.14% 🔽)
@sentry-internal/feedback - Webpack (gzipped)	17.12 KB (-0.48% 🔽)