fix: correctly detect Cloudflare Workers when either main or assets key exists

[eltigerchino]

fixes #13832

This PR ensures that if the main key is found in the Wrangler configuration file, that's enough to know that we're building for Cloudflare Workers. Previously, it would continue to check if the assets key exists too. However, the non-existence of the assets key doesn't mean we're building for Cloudflare Pages. It simply means someone has forgotten to add the assets key if they have specified the main key.

I'd still like to add some unit tests for this so this is not ready yet.

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Name	Type
@sveltejs/adapter-cloudflare	Patch

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