feat(sveltekit): Ensure AsyncLocalStorage async context strategy is used in Cloudflare Pages

[Lms24]

While investigating #15506, I noticed that we missed an important part when setting up the SDK via the initCloudflareSentryHandle hook: We need to call @sentry/cloudflare's setAsyncLocalStorageAsyncContextStrategy function as otherwise, we don't get working request isolation.

I decided to export setAsyncLocalStorageAsyncContextStrategy from @sentry/cloudflare so that we could otherwise only call sentryPagesPlugin and completely ignore the return value. Which isn't ideal as we add public API. However, initCloudflareSentryHandle is basically a SvelteKit-specific version of SentryPagesPlugin already, so I think it's fine to just set the ACS in there as well.

If reviewers prefer though, I think we can solve this in other ways:

1. export setAsyncLocalStorageAsyncContextStrategy from @sentry/cloudflare/internal
2. call sentryPagesPlugin and completely ignore the return type

Happy to make any of these changes.

Otherwise, the PR adds tests and refactors the test directory structure after #13142.

note: feat because public API is added to cloudflare package

[AbhiPrasad]

Keeping it as public API feels fine to me because of all the use cases, lets do it.