Read mode/credentials from init object, not request

[Rich-Harris]

This is an alternative to #7192. Rather than checking for a cf property, it reads mode and credentials from the second argument passed to fetch, unless fetch was called with a Request object (which is a somewhat unusual case; code that does fetch(request) would have to be refactored to fetch(url, init) on Cloudflare).

I'm slightly ambivalent about which approach is better. This gets rid of the Cloudflare-specific cf stuff, and in theory is more resilient to other runtimes doing the same thing, but it would force the aforementioned refactoring in a very small minority of cases.

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[dummdidumm]

I'd say we go with this first and then see how often it comes up afterwards? We can always add the other PR on top/instead later on.