fix(sveltekit): Flush in server wrappers before exiting

[Lms24]

When deploying to Vercel (non-edge), server code is converted into Lambda functions. These functions have a tendency to freeze/shut down before the SDK is able to flush out events, causing events to be dropped. We already handle this in NextJS by manually calling flush. This PR applies the (almost) same logic to SvelteKit's server side instrumentation.

Since I was already at it, I replaced the trace calls with startSpan which is the newer API.

fixes #8855

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	84.24 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.41 KB (0%)
@sentry/browser - Webpack (gzipped)	22 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	78.69 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.52 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.59 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	254.14 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	86.42 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	61.23 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.38 KB (-0.01% 🔽)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	84.27 KB (0%)
@sentry/react - Webpack (gzipped)	22.05 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	102.23 KB (-0.02% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	50.98 KB (-0.04% 🔽)