ref(sveltekit): Handle SvelteKit-generated spans in sentryHandle

[Lms24]

(This is still WIP and we need more follow-up PRs afterwards)

• Don't start a span if the kit request event signals that Sveltekit emits its own spans
• We still isolate the request but this should work correctly. We'll see..
• Add tests

see sveltejs/kit#13899

ref #16982